This guide has two parts.

In Part A, I make some general observations about various aspects of the game.

In part B, I outline some of the features of the playable factions, and suggest research approaches for each.

Other approaches may be just as good, or better, and constructive comment is welcome.

1. Quests
The ingame quests and the Lord of Skulls option are challenging to say the least. It might be worth starting with those options disabled (uncheck 'parameters' advanced and DLC when setting up a new game) to avoid unnecessary annoyance whie you are still getting to grips with the basics of the game.

2. Loyalty
Always keep an eye on your cities' loyalty (the red flag resource) and don't let it drop below 0 or your empire's productivity will quickly grind to a standstill. Each faction has a building that specifically increases loyalty.

3. Production and Base Growth.
With all the fighting going on, it's easy to concentrate on military production and overlook other things. You start with one labour building, and one infantry building; the machine building, the hero building and the airport have to be researched. If you find youself needing a lot of, say, infantry, it may well be worth building one or even two extra infantry buildings in the same base to speed up production. This is particularly true later on when 3 airports will enable you to spam planes in great numbers.The labour building can also be duplicated to speed up tile acquisition and base development - sacrificing a bit of time early on for much faster growth later.

4. Every game is different.
A lot depends on where you start - what bonuses the tiles provide etc. and where you are placed strategically. If you get crushed quickly by an AI player, don't lose hope. This happened to me in my 5th or 6th game, after I'd already won several times - that's just how things are sometimes. It may even be worth setting one of the AI players as an ally (available when setting up a new game) to give yourself a bit less pressure at the start., and also to see how the AI handles strategy and tactics.

5. Every game is the same.
(a) The independent (grey) units are super aggressive. When exploring expect the worst and either reconnoitre in force, or use only units you can afford to lose.
(b) Terrain, particularly forests, is hugely important. You can rack up easy kills by setting up your forces to trap opponents unawares. P35, heading 4.1 is by far the most important point in the whole game manual. Also watch out for obstacles such as cliffs and escarpments which can greatly effect movement.
(c) Why can't I start a new base? There are restrictions on Space Marine and Necron bases, discussed below. However, in addition to these, YOU MUST HAVE ENOUGH RESOURCES! The price gets higher the more bases you settle. But if you want to settle, and can't, check you are not too close to another base, and have enough resources.

The game requires you to research two techs (out of 5 or 6) at each tier, before moving on to the next tier. As the really powerful units come later on, it's worth considering how to get to higher tiers fastest with the best research. Other research can also be very useful, and there is not only one way to go about it!

Each type of army has a main purchase cost (what it costs to buy units etc.) and a main support cost (what it costs to maintain those units). It's helpful to know what these are when deciding how to develop your base tiles.

The SM are only allowed one base - (Space Venice!) - so the above observations on Production and Base Growth are particularly important. The SM have the ability to drop fortified outposts on any visible tile once every 10 turns, which can be a useful situational ability at times.

Must-have units: Devastator Space Marines, Apothecaries.

Why? Devastators have a huge damage attack when they have not moved, but they are squishy, so you want something in front, namely a regular marine. The Apothecaries provide the necessary healing ability. Unfortunately they arrive quite late, so be prepared to churn out a lot of basic SMs as cannon fodder meanwhile.

The SM's basic weapon is the bolter.

Main Purchase Cost: Requisitions
Main Support Cost: Requisitions, Energy.

Core research tree.
Tier 1 Armory, Orbital Scan
Tier 2 Dormitories, Krak Grenade
Tier 3 Chapel, Devastator Space Marines
Tier 4 Orbital Relay, Kraken Bolts
Tier 5 Extra Infantry Armour, Apothecaries
Tier 6 Thunderfire Cannon, Signum

Note that the Thunderfire Cannon can repair machines and clear tiles as well.
Note that you will also need the Fortress Expansion at some stage so as to expand your base.

Updated July 2022

The NK can only build bases on 'tomb tiles', which have a green swirly effect on them. The Canoptek Spyder is the 'settler' unit.

Must-have units:Tomb Blades (early) Canoptek Spyder, Monolith (later).

Why? Tomb Blades have great movement and a spectacular early melee-range attack but you need to wipe out the enemy on first contact, as they are easily killed. Build a squad of Tomb Blades and send them off to explore. or hold strategic bits of forest. Just bear in mind that anything they can't kill straight away, will almost certainly be able to kill them on its own turn. As stronger units emerge, the Monolith is your friend. A huge attack with its Gauss Flux Arc (note: this needs to be selected each turn for some reason), and huge health to match. The Spyder is a support/settler unit but it has a respectable melee attack and it can also repair machines (the Monolith!) once that tech is researched. Plus it can spawn swarms of scarabs every 5 turns which are fast, lethal and expendable.

The NK's basic weapon is Gauss.

Main Purchase Cost: Energy
Main Support Cost: Ore

Tier 1 Core Refinements, Hypostyle Temple
Tier 2 Shield Vane, Shelter
Tier 3 Baroque Shrine, Canoptek Spyder
Tier 4 Preliminary Excavations, Slave Mastaba
Tier 5 Extra Infantry Armour, Atomic Flayers
Tier 6 Doomsday Ark, Cryptex Datastyles
Tier 7 Monolith, Nameless Causeway (For Doom Scythe in Tier 8)

The Nebuloscope is also avaliable to imporve Tomb Blades, if desired, but returns do diminish nevertheless. Another possibility is to go with Flayed Ones in Tier 1 instead of the Temple, which has the benefit of being an early pure infantry build. To be honest, Tomb Blades are just way more fun...

Updated to July 2022

AM don't have any building restrictions. The game will offer tile recommendations (the white tiles) within range, but they are optional.

The AM's basic weapon is the laser.

Must-have units: Enginseer, Basilisk, tanks.

Why? The Enginseer is the settler unit, and can also repair machines and clear tiles of forest etc. As a fighter it's not in the same league as either the Canoptek Spyder or the Thunderfire Cannon, but nobody's perfect. Last but by no means least the Enginseer can acquire the ability to build the Imperial Bastion, a very useful fixed defence. The Basilisk has an amazing range of 6 tiles, from which it can make innumberable kills. The Tank Commander is an early powerful unit which can hunt with a Leman Rus pack searching for targets for the basilisks to finish off.

Main Purchase Cost: Food/Ore
Main Support Cost: Food/Ore

Tier 1 Manufactorium, Shrine of the Aquila
Tier 2 Cult Mechanicus, Medi-Pak
Tier 3 Heavy Weapon Squad, Tank Commander
Tier 4 Basilisk, Hotshot Powerpaks
Tier 5 Imperial Bastion, Leman Rus Tank

Again, other routes are also possible, e.g. levelling up grenades, producing psykers, if desired.

The Orks basic weapon is melee.

Must-have units: Boyz, Meks, Battlewagon, Dakkajet

Why? Boyz are the basic infantry unit, but they remain effective for many turns. Meks (the 'settler' unit) have 2 functions: settling new bases - and Ork urban sprawl is a good thing - and later on repairing machines. Continuous build these units at the start. As the Boyz begin to fade, switch to building battlewagons; they are tough but require large numbers until their damage is upgraded. The keystone unit is the Dakkajet whch doesn't require an airport. 3 or 4 are quite capable of taking out a base on a couple of turns.

Main Purchase Cost: Food/Ore/Energy
Main Support Cost: Food/Ore/Energy/Influence

Orks seem to have more loyalty problems than most, and they have a variety of cost requirements. It's particularly useful to get a second labour building (Scrap Shedz) in your bases early so that shortfalls can be addressed promptly. Other than that, spam spam, spam, units and apply fungus to your base tiles when you are waitig for other resources, as it heals your garrisoned units faster.

Tier 1 Kult ov Speed, Orchid Fungus
Tier 2 Grotz Homs, Mek Boys Yard
Tier 3 Eavy Armor, Fighting Pitz
Tier 4 Bigga Choppas, Scrap Shedz
Tier 5 Battlewagon, Red Paint Job
Tier 6 Battlewagon Big Shootas, Freaking Rooms
Tier 7 Dakkajet, Extra Vehicle Armor

There is a different route where you can build Warbosses and heal them with Painboyz.

Tyranids don't have any building restrictions. The game will offer tile recommendations (the white tiles) within range, but they are optional.

The Tyranids' basic weapon is their brute force!

Must-have units: Tyranid Prime, Carnifex, Tervigon

Why? The Tyranids game mechanics are different from the other races. You start with Termagants, which are good little units (importantly they are good against the wild flies) but need a more evolved unit to stop them losing morale; otherwise they can't stray far from their base. So you will want to start by recruiting some Tyranid Warriors to shepherd them about. The warriors in turn fight much better with a Tyranid Prime Hero nearby. The Carnifex is literally a beast - a couple can kill most things- but it loses health if it is not near a Hero. The Tervigon is another Hero unit. It does 2 things: it heals other units every turn - vital when they concentrate on melee - and also spawns Termagants.

Main Purchase Cost: Biomass/Influence
Main Support Cost: Biomass/Influence

Tier 1 Alpha Ovipository, Tyranid Warriors
Tier 2 Aedanthropum, Tervigon
Tier 3 Brood Haunt, Coherence Node
Tier 4 Metamorphic Malanthropes, Ripper Dispersion
Tier 5 Megafauna Sac, Extra Infantry Armour
Tier 6 Aggressive Expansion, Exocrine.

The strategy here is to assemble an initial force of 2-3 Termagants, a couple of Tyranid Warriors and a Tyranid Prime before expanding aggresively. The next unit is the Tervigon, at which point you are set, because it can keep your Tyranid Prime alive more or less indefinitely. As soon as it levels up it can also spawn dispensible Termagants to cover your retreat in the unlikely case something goes wrong. There is no rush to start a second base, but once you have the base buffs, you may want to build a couple quickly to catch up. You need the Malanthrope from the Aedanthropum for this. The Exocrine rounds out the group with its fabulous range 2 attack though it is also excellent at melee once you have the techs.

Generally I found Tyranids quite OP once they got going. A decent Tyranid Prime Hero with a couple of Carnifexes, constantly healed by the Tervigon, can take out most early AI bases. The Exocrine turns the process into a slaughter. The Tyranids main early weakness is air (Psychneueinn Flies) so don't forget to take enough Termagants when exploring.

The Eldar have a unique mechanic, in that they start with various possible building locations, called Webway Gates, but these can be, and generally are, a focal point for destruction by every other faction in the game. So by say turn 20 most possible locations will already have been destroyed. The other issue is that if you do claim a gate, and build a redoubt (which is admittedly is quite good at defending itself) the redoubt counts as a city for loyalty purposes. Unless you are prepared to play as Elf Space Marines, you therefore take a double hit in the early game. But that's not all. Eldar populations grow very slowly, so you are both taking that hit and not in a position to develop your cities quickly to negate it with loyalty buildings.

So the Eldar are faced with several early and important strategic/economic decisions. You need to consider various things:
(1) Early units require the food resource. If you're given a start with a lot of bad food tiles, then it might be worth restarting.
(2) After food, you will need energy, and influence. Loyalty is also important, as explained above.
(a) energy, for all the usual reasons
(b) Influence because you need it to use your base-building and teleporting powers, obtain Spiritseers (see below), and activate Asuryan Arrivals and if needed Transcendent Bliss
(c) Loyalty because otherwise the Eldar's already sluggish economy will slow to a complete halt.
(d) Not to mention ore, though I haven't found that a particular problem in my games.
So, basically you will be short of everything at once, and that will be the case for a while!

To add to the fun, your units are also unusual. The blurb for the DLC claims that 'Aeldari units are absolutely lethal from a distance, but you won't lack for deadly close combat options either.' Not really. The starting units (Guardians) are ranged, but the range is 1. The Rangers have good range(3), but practically no armour piercing. In practice this makes the Eldar quite deadly against wildlife, not so much against other factions. You will need Howling Banshees early on to cope with anything armoured, and they are not particularly well-armoured themselves. which brings me to:

Must-have units: Spiritseer, Howling Banshees, Vaul's Wrath Support Battery.

Spiritseer - The good news is that Spiritseers can heal any unit you have; the bad news is that they are a hero unit. Yet another claim on your precious influence, and one that escalates into the bargain, because you'l likely need at least 2.
Banshees - for all their faults they are your only real early counter to anything armoured. Be prepared to lose them though - various units can simply one-shot them in melee.
Vaul's Wrath Support Battery - good unit, fires over obstacles, and can destroy enemy bases quite handily.

In the later game I have mainly used Fire Prisms (Tier 6) and Crimson Hunters (Portal Spire, Tier 7). War Walkers are excellent, but they don't come until Tier 8.
So what about the Avatar of Khaine? Devastating unit, only needs Tier 4 research? True, but he is super expensive, and because of how the Eldar work, you can either have the Avatar or an economy/ability to jump between cities. Economy/ defence first - later on by all means the Avatar.

Main Purchase Cost: Food/Ore/Energy
Main Support Cost: Food/Ore/Energy
But the Eldar do not live by Food/Ore/Energy alone - see above.

Getting started:

Tier 1 Webway Cartography (because you can claim Webway Gates for free), Rangers.
Tier 2 Spiritseer (also requires Shrine of Khaine), Howling Banshees
Tier 3 Cairn of Ulthanesh, Vaul's Wrath Support Battery (you'll need to slot in Greater Ingress somewhere to claim it)
Tier 4 Bonesingers Chantry, Exodite Foothold. (About this stage you'll need to pick up Ghost Halls to expand, and Transcendent Bliss if Loyalty is an issue)
Tier 5 Labyrinthian Mesh Armour, Farseers Summons. Wave Serpent is not a bad unit, but you're getting Fire Prism at Tier 6, plus anything that saves influence is important to Eldar.
Tier 6 Fire Prism, sub-molecular planes.

Strategy.

The Eldar are quite strong right at the start. Churn out Guardians and Rangers and teach the wildlife who's boss. You'll need to decide when, where, and whether to build additional Webway Redoubts. I suggest one, max two, or you'll never recover from the loyalty penalty.

For all their many faults the early Eldar units have one huge plus, Battle Focus, i.e. fire and move. This makes them masters of the 'strategic withdrawal to previously prepared positions'. Plan for foward defence with room to retreat to cover. Don't forget Banshees to cope with armour. Be prepared for the defensive phase to last quite a while while your cities grow at a painfully crawling pace.

Finally - payback time. The early game is tantalising yet frustrating for Eldar, because so many goodies seem almost in reach, yet there are so many competing demands for resources.
Do not despair.
You will get the Avatar.
You will get the Autarch and the Farseer Skyrunner, both perfectly good units but for the influence bottleneck.
You will get War Walkers.
You will get the Scorpion.
And your foes will pay the ultimate price for destroying your Webway Gates.

Personally I'm not keen on the Webway Gate destruction mechanic, whose animations slow the game right at the start when the player is keen to crack on with things. On the other hand, it presents the Eldar with interesting and unique challenges about what to try to save and at what cost in terms of later economic development. If we could keep the one and get rid of the other, the Eldar could well be the best faction in the game.

I'm sorry, but I just can't like Tau. However, help is at hand in the form of this exemplary guide from Adozu:
https://steamproxy.net/sharedfiles/filedetails/?id=2385071647

CS start with a Chaos Space Marine and 2 Chaos Cultists, a basic melee unit that also happens to be the build city/ clear tiles unit of the faction. So in theory CS could have two cities on turn 1. Avoid the temptation, and just settle one.

CS effectively have an additional unit tree - chaos - offering a variety of possible starts; but this also means that you can be short of population if you try to build everything at once. Note here that in addition to requiring a wide range of resources for their (expensive) units, CS can also have loyalty issues, so every building slot matters. I found that it helped, rather than building 2-3 infantry buildings as usual, to have cultists and marines building in parallel in just the one building each.

Cultists have a reasonable attack against weak wildlife, but poor defence. They also have a special ability - on killing a unit they can randomly mutate to a better unit. In my first game I got a Demon Prince this way, but never since, so it doesn't seem very reliable. Nor are the Chaos Space Marines as good as their Dudley Doright counterparts.

With these points in mind, I suggest the strategy outlined below.

Must-have units: Master of Possession, Havocs (early), later on, mech.

CS are the only faction where I would start on a hero building as soon as it becomes available. The reason is the Master of Possession. For most factions, wildlife is a major headache. For CS though, it is a farmable resource. Every time your Master of Possession kills an enemy, it is replaced by a Chaos Spawn (you don't even need to research the tech, and I never have).

Given that they're free, Chaos Spawn are pretty good units, with a decent melee attack and great mobility. Unfortunately they don't have a ranged attack, which is where the Havocs come in. Like other heavy weapons units, they have a range 3 attack which is enhanced if they have not moved. Chaos Spawns and Havocs thus make a nice combination that should see you through the early game.

Once you complete the research order below, you will want some of the better (i.e. mech) units on the CS roster. Forgefiends and Helbrutes are good in the Infernal Fabricator, and and Heldrakes despite being effectively melee are formidable air-to-ground units, as well as anti-air.

The CS's basic weapon (apart from melee) is Bolter

Main Purchase Cost: Food, Ore
Main Support Cost: Food

Build Order
Tier 1 Abominator's Genelab, Lord's Quarters
Tier 2 Captive Pens, Master of Possession
Tier 3 Havocs, Meadow of Sorrow
Tier 4 Warpforge, Warpborne Bolts
Tier 5 Extra Infantry Armour, Call of the Dark Gods
Tier 6 Forgefiend, Conjoined Weaponry

Naturally, the Infernal Fabricator (Tier 1) needs to be researched somewhere along the line, possibly right before Tier 6.

The various Marks, Boons and Icons can also extend the useful life of your infantry, if you are that way inclined.

July 2022

Adepta Sororitas (Sisters) are in some ways a 'vanilla' faction, reasonably good at most things, but not (superficially) excelling at anything. Certainly, they are not just a 'reskin' of Space Marines. The first couple of games I played with them seemed to be defined by the mechanics they lacked, but - like all factions - they need to be played on their own terms, and can be quite good fun.

The Sisters can build a main base, and small satellite outposts with just the first ring of tiles. Optimal building of the outposts is virtually a minigame in itself, so perhaps more than any other faction, Sisters need a decent starting location, with space to grow and Requisition and Research available early. You can then plan your outpost (I only ever needed one) as a power station. Tip: Don't build your outpost beyond 12 pop, and remember you can switch off buildings you are not currently using in it.

Next, Sisters need rapid research, as they tend to fall behind in the midgame, before their killer aerial artillery can swarm. Building extra research is possible not least because Sisters gain loyalty for each tile claimed in a city, meaning less flags, and so more books.

Must-have units: (early) Battle Sisters, Hospitallers, Celestine; (late) Avenger Strike Fighters.

Why? As a faction, for most of the game, Sisters are all about the Sisters themselves. The Battle Sisters unit has good stats, and once you get the Hospitallers healing them are virtually impossible to kill if a group is defending a treeline. The range they lack can be remedied in the earlyish game by the Castigator Tank. The Sisters' hero unit Celestine is even tougher, and has a decent attack. Their late game is best played with the deadly Avenger Strike Fighters en masse, protected by a couple of Lightning Fighters, as they are weak against other air units.

Like SM, the Sisters's basic weapon is the bolter.

Main Purchase Cost: Requisitions
Main Support Cost: Requisitions, Energy.

Core research tree.
Tier 1 Legatines' Priory, Anointed Hangar.
Tier 2 Cells Urbis, Krak Grenade
Tier 3 Hospitallers, Famulus Mission
Tier 4 Ministorum Workshop, Preceptory Annexes, Celestine
Tier 5 Extra Infantry Armour, Castigator.
Tier 6 Dozer Blade, Sacral Vigor.

After that, sprint for Tier 9, grabbing Ecclesiastical Reinforcements for the airports at tier 7. The battlesuit units are good fun, but the Avenger Strike Fighters inherently have much better movement.

Note that the Hospitallers can clear tiles as well, but NOT repair machines- a general weakness of the faction.