Okay, the defense of my fortress relied heavily on traps, especially cage traps. This can be noted by looking at the animal stockpile which is full of goblins and trolls. First there is narrow walled route filled with traps, which leads on bigger room, where my military dwarves are standing in the picture. That's my first "waiting room" for military. During ambushes, megabeasts and such I order them to station there to wait action. That's also the position where they station automatically by scheduled orders. Again some narrow passages filled with traps and you come to second "waiting room" right before bridge. This has not been used in a long time. Over the bridge and there comes the last room where my military will wait during sieges. To get that room invaders have to pass all the traps, thinning the forces nicely.
The water reservoir visible in picture is part of the drowning chamber trap. Never had any need for it.
For military, I have tree squads: Military, Guard and Military 2. The have different names inside the game, But I used those names as custom professions when drafting.
Military, consisting of 5 axe dwarves and my Military Commander, defends the main entrance. They are usually stationed in Waiting Room 1, though the spot may differ. I have assigned uniform consisting of full adamantine suite and battle axe to this squad.
Military 2 is very similar as Military, though they are stationed at the cavern entrance, leaded by captain and they have equipment made from steel.
Guard squad is lead by Captain of the Guard, including 5 guards as well. Steel equipment, they have patrol route between the cavern entrance and main entrance,
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This is represents the efficient of my Military squad, as most of them are more or less same amount of kills. Several times I have seen only one soldiers chasing the whole siege away, while rest of the squad has been drinking/sleeping. So far only two soldiers appointed in this squad has died, first one drowned to the river and second one fell in battle after chasing the whole group of goblins away. No-one ever carried him to the hospital, and the wounds were to bad.
Lastly, in field of defense, I have burrows set to cover my dining room and food/drink stockpiles, and when the siege comes I order every civilian to that burrow from military -> alerts -screen. Seen too many times one of my mechanics dying when deciding to reload a trap during siege. Which reminds me of the fun fact, besides being in military, mechanic is the most dangerous single profession in my fort. After all the soldiers, the most deaths has been in that profession.