Whew, this is a nice and challenging TD game! It feels impossible at first, but is anything but impossible, just needs a decent amount of strategy and it's completely doable, even easy.

Here are some things that i've noticed during playing this awesome TD:

1. Keep up the damage all the time: Especially the tanks have massive defense and hitpoints. The damage should be dealt constantly instead of a few high-level towers here and there. Flametowers and lightning towers are great for this. Flame burns for long, but the slow tower extinquishes it, so don't put them next to each other.

2. The flying annoyance: Air force on Nightmare-dfficulty is usually MASSIVE and fast, and it takes many missile towers to keep then from reaching to the exit. Whenever possible, put missile towers near the starting points of the planes first and upgrade them as a priority. They take off many planes right at the start of waves.

3: Make spaces: Use the massive weapons to create spaces between enemies on the long waves, this helps out surprisingly lot to ease the pressure for a bit.

4: Priorities and hell fire: Each tower has their own priority, don't hesitate the change them when needed. Optimal target is the best imo, since it minimises to firing time, but sometimes the attention is needed elsewhere. I also noticed that keeping the hell fire on is better than manually activating it, especially in the end. Reduces the hassle greatly.

5. Slowing towers vs flame: I was a bit torn between the slowing towers and inferno towers, but found that the slowing ones combined with tesla coil was a much better crowd control-combo than a flame towers on it's own. Slowing towers don't target ships, so flame towers for them!

Being the first level and couple towers to choose from, it's simple. Wave 4 brings on the cars so you need some missiles to combat them. There's not much else to say in this level so let's move on!

Here things get a bit more interesting. We don't have much money in the whole level so the big guns and mines are our friend! Place mines along the end of the path, to counter if someone gets through. The "lower circle" at the bottom of the map will be the main place to destroy air force, while the middle circle will finish off hte land forces.

Wave 1:
Start by putting up two lasers so that they shoot both routes and upgrade as possible.

Wave 2:
Put a laser under the air force path, so that it intercepts them when they arrive, and upgrade it. It's enough for now concerning the planes.

Wave 3-4:
Put up a missile launcher amidts the middle circle, right on the inner edge, so that it eventually start to shoot at the fathest path also.

Wave 5:
On the lower circle, place a missile tower in the middle, don't worry about the small range, it's going to be upgraded. Also add laser towers in the center of the middle circle.

Wave 6: This is a tough wave with the current towers! Use the special weapons to break the big line of enemies coming, create an opening here and there to ease the pressure and upgrade the missile towers.

Wave 7 - 15:
Keep upgrading the missile tower on the edge at the middle circle, the lasers, and add another to the bottom circle. The air force is getting bigger so we need more power down there. Keep upgrading the existing towers evenly.

Wave 16:
Add 3 lasers towers and upgrade them.

Wave 17:
This is where i did the one mistake, i forgot that i should not upgrade that laser tower that helps with the air force, DO NOT DO WHAT I DID, DO WHAT I SAY!

Wave 18 - 20:

Rest of the level is quite a smooth ride, upgrade and use the special weapons as necessary.

More towers and bigger special weapons! Now we're talking. There are two important things to keep in mind on this stage:

• The flamers at the beginning of the path. Without them upgraded to the max at level 17-18 you will lose. The last upgrade takes A LOT OF TIME, so everything else needs to be strong already to hold.
  
• The missile towers situated at the airforce path. These both points have to be upgraded pretty evenly to ensure victory.

Wave 1:

The first wave comes from the upper path, one flamethrower to toast them all.

Wave 2:

You get just enough of money for another flamethrower, it's a sign, so another flamethrower to toast them all from the lower path. Right near the end of the wave, upgrade the upper flamer.

Wave 3:

Soon we have again the air force at us. Put a missile launcher at the middle, intercepting three paths and another one at the highmost circle, near the air route, upgrade them both.

Wave 4:

Upgrade the lower flamer after the wave has come, and add a missile tower at the roof of the southwest building. THIS IS AN IMPORTANT ONE, it picks out many airplanes right at the start of their route, and prevents them from bothering the turret towers higher up, which are concentrating on land forces.

Wave 5:

Nothing else than upgrading the upper flamer.

Wave 6:

Add two missile towers in the middle, next to the one that's already there and upgrade.

Wave 7:

At the end of the wave, upgrade the bottom flamer.

Wave 8 - 11:

Continue upgrading the towers evenly.

Wave 12-15:

12 is the wave where we start to deploy the boss killers.. ahem, i mean the howitzers. They also act as a support while we're upgrading the upper flamer to the max level. Deploy a Howitzer and a slower next to it. Also put another Howitzer at the bottom circle, and slower around it. Upgrade missiles

Wave 16:

Time to upgrade the upper flamer near the end of the wave.

Wave 17
When the falmer has finished upgrading, make to howitzers a bit stronger. Now is the last moment to upgrade the bottom flamer. Tanks are getting too strong to withstand otherwise.

Wave 18 - 20:

Now that the flamers are fully upgraded, it's time to max out the rockets. Massive air force is coming so they need to be ready.

Wave 21 - 25:

After the rockets are maxed out it's time to start getting ready for the bosses. If you haven't yet, deploy two howitzers at the woods on the lower path and one on the last circle. Put slowing towers on their places and upgrade everything evenly. Then sit back and watch the bosses fall.

- the long straight in the middle is a perfect chokepoint. Lots of AoE possibility on that one, so i decided to focus my damage in there on this level.

Wave 1 - 2:

Put up tesla coils at the north and south junctions. Upgrading them on wave 2.

Wave 2 - 3:

Start putting up missiles, level 5 brings the first air assault. On the left side, remember to leave a 5x5 grid for the slowing tower later on. This one is important! Also put up a tesla coil near the end of the fifth wave.

Wave 6 - 12:
This is a bit tricky, even with the upgraded tesla coil, you might want to put up the firewall on the bridge, just to be sure. Build a few more missile towers, and keep upgrading steadily all over the level, missiles are important, but don't forget the tesla coils!

Wave 13 -15 :
Put up a tornado below the bridge and fill the remaining space with a missile tower. Keep upgrading the existing towers and at around wave 15 start adding the slowing towers to the middle and at the bottom.

Wave 16 - 30
It goes pretty easily from here on. Keep adding slow towers here and there, try to get as much area covered in liquid nitrogen as possible. Keep upgrading the existing towers evenly. Adding a few howitzers in the middle and it's quite smooth sailing towards the flawless victory.

-The bridge on the right is going to be our main damage dealing spot on this mission.
There will be air units that survive the first missile section, so be sure to put up missiles that target only air units at the north side of the river!

Wave 1 - 2:
Start by putting up a tesla coil on the northwest side of the bridge, and change the preference from tank - ship to ship - tank. When you have the money, do the same on the southeast side of the bridge. Upgrade the tesla coils right at the end of wave 2

Wave 3 - 4:

Put up a missile on the south side, and upgrade it once, this is enough tot ackel the first air assault. and make sure you upgrade the tesla coils to level 3 during wave 4.

Wave 5 - 6:
Either one of the tesla coils should be upgraded to level 4 before wave 7. Otherwise the cars will get through.

Wave 7:
Wave 8 sends more air units, you need more missiles to tackle them. 2-3 level 2 missiles are enough, especially if you have one at the north side to tackle the survivors.

Wave 8 - 10:
Wave 11 and 12 will send more air units, 12 will send a powerful ones, so upgrade the southern missile section and build more missiles. Around wave 9, build the slowing towers on the bridge. This will make a massive difference.

Wave 11:
It's time to start adding the damage before the bridge. So set up a tesla coil at the halfway of the upper path with a slowing tower.

Wave 12:
More missiles at the south section! Also change the upper missile batteries the plane only and leave the lower ones to target everything. Add missiles to the north side as well.

Wave 13 - 22 :
Add the 3rd and 4th tesla coil on the bridge, and upgrade them. Modt of the towers are now in their places. Keep upgrading them evenly, trying to get the teslas up, and increasing range on the slowing towers. You know you're good if most enemies die on the bridge.
Also add a tesla at the south path, in the middle with a slowing tower.

Wave 23 - 33.
Start adding Howitzers near the bridge and keep on upgrading towers evenly. There's not much to do in here anymore than to upgrade and add two tornado's around the middle that can shoot EVERYWHERE!

Wave 34:
NOW THIS IS WHERE IT GETS A BIT TRICKY! The mechs are durable, and they're coming from both ways. When they're on the bridge. just unleash the needed special weapons to kill mechs from both routes et the same time.

Wave 35:
The boss is much easier than the wave before it.