Now when to make a moat? when you have resources close-by and easily possible, unless it is a weapon moat... A moat provides for excellent basic defence, it is quite aestheic and looks fantastic and eye soothing. A water moat provides water for a lot of purposes. Other two are basically very, very good defence strategies. Like legit enemy melting methods. Water moat just sinks the enemy. Direct death. Just that it needs to be kinda broad, 4x4 or something. But mainly it acts like a fortress and map isolator, just the land and not the sky.

See there are three types of moats, namely

• The Normal Water Moat (very easy and efficient, good for aesthetics)
  
• The Magma Moat that requires bulk magma pumping (tedious yet rewarding)
  
• The Weapon trap Moat (really rare)

Water Moat
You can make a water moat very easily and efficiently when you have a river, creek, ocean, waterfall ,waterbody nearby. There is a procedure which will be explained in further sections in detail for safety and well being of the dwarves (if you want to)

Magma Moat
A very deadly and fun moat, advanced stuff... very advanced micro-management and automation can be implemented. Unless you just make a pump stack and pump it straight in or have a live volcano on embarkation. I won't go on this in much detail here but I will give a blue print so one can easily implement it.

Weapon Moat
You make a Moat, empty, yet full of weapons. So when the enemy is walking along the bridges towards your fortress.....

• There are walls on both sides, retracting bridges under the feet of enemy.
  
• when entire army on bridges ( bigger layout)
  
• Retract bridge! (unless it is a massive titan or a hydra or something like that)
  
• GUESS WHAT HAPPENS WHEN EVERY SINGLE BRIDGE RETRACTS?????
  
• ???
  
• Tremendous amount of fun for the enemy army!

So yeah thats the weapon moat concept for you... actually comes under the dwarf-fortress weaponry tutorial but yes, it also classifies as a moat so...
I shall go on depth in the Water Moat in this guide. The magma moat and weapon moat need to be actually played and screen-capped a lot to make a guide on.
Oh and by the way, I made a nice little GIF file, you can have it:

A water moat should be made,

• If you are new then there should be water easily available.
  
• If you are experienced, wooden pump stacks do the job as well, quite easily.
  
• If you have a: River, Ocean, Waterfall, Stream, Water body of any manner.

Now don't make a water moat where throught the year the water is frozen. That doesn't end well in real emergency.

Now this is a general representation of how a moat should be made but I'd say this is a large moat.
You can see here, my animals are safe, there's a fishing zone, there's bee farms, multiple by-passes. This is a good moat. Keeps all the stuff safe and sound. I even got many trees that give fruits and dwarves can just gather the tubers and fallen fruit to satisfy the wander need.
The important thing here is the walls. Walls are critical in any good fortress. They direct the flow of enemies towards you. We even have a water wheel that runs some mills. That's the general concept and description of a moat. How it helps and all.
This is a fishing area with a bypass.
On the Left is Z1 and on the right is Z2, so you can see how the fisherman can easily fish due to the water and a bypass. You can legit close water on the left side with bridges before digging into to open the moat. The wall is the primary defence.

This page explains the layouts:
Here you can see the moat layouts, the water drop trap layout and the advantages of moat.
The page will also tell you about the basic digging out and layout of the moats. All the bridges are obviously linked by named levers in a lever control room.

This should depict the safe and sound water wheel mill-stone.

The concept of river drainer is going to be a seperate guide but I will give the design with some images here.
This is the River Drainer.
This is the River Drainer in action.
This is the water Side view of River Drainer.
This is the Bigger Design and this is the water side in action.

I hope these pages explain whatever is there to this sector of the moat guide.

Work in Progress