Layer one of the defence of Claspedchurches: The curtain wall that extends around my entire 5x6 embark.
2: One of my three 10-dorf-fully-legendary-clad-in-adamantine murder squads is on patrol around the outside of the walls at all time. (the second trains, ready to respond to hot-spots the third is stationed in the deeps to protect my workers down there)
3: Inside the entrance to the curtain wall is LOOOOOOOOOOOOOOOOOOOOONG corridor protected by siege engines manned by siege operators who have been practicing since the dawn of time.
4: Past the siege engines lies the LABRYNTH. Full of cage and weapon traps.
5: At the exit to the LABRYTH sits the statue head of Armok. A giant stone head that forms the picture of the warrior dorf ascii with 1 block == 1 pixel. The statue can spit lava and cry water to obsidianify all foes who emerge from the labrynth unscathed.
6: Past the giant head of armok is another tunnel. This one full of more weapon traps and cage traps. At the end of the tunnel is another ballista and BEARMAGEDDON. A cage full of war black bears, war grizzly bears, war dogs, war alligators, and war crundles. Over 125 animals all told.
7: Should the enemy get past all these defences, they shall emerge into the fortress's outer cortyard ie: Most of the outside map. Outside farming, tree harvesting, herbing and fishing all take place here. However, to take control of fortress Claspedchurches, they must still breach the inner curtain wall, its associated moat, drawbridge, and archery tower.
8: If by some unholy miracle the enemy has made it through to the inner courtyard, they will find themselves facing the rest of the military as they emerge from their danger-room-barracks.
9: Past the barracks is the entrance tunnel into the fortress proper. Said tunnel of course has a line of cage traps surrounding it. Past the cage traps is a 4x8 self-cleaning and self draining drowning trap.
10: Past the drowning trap is another long hallway tunnel full of weapon and cage traps and some stone-fall traps from the fortress's origins that I haven't yet got around to replacing cuz there is no need. The end of the hallway has fortifications protecting a ballista that can fire down the whole length.
11: Only two chained war bears stand between the civilians and DEATH at this point.
12: civilian slaughter time for anything that managed to get this far.
To date, I have only ever needed the drowning trap once, way back when it and the very final tunnel were my only defences. Since then, my military has just murdered everything, so most of my defences of DOOM remain untested.