Clusteredhalls was going quite well - built at the base of a large volcano, full of magma to about 90 levels above me, magma forges churning out bronze goodies and everyone safe as I'd walled off a large portion of the outside world and lined the land outside that with bridges and cage traps. No pits, just bridges and dozens of levers.
Worked quite well. Several large sieges ended when the goblins ran through the gauntlet of raising and lowering bridges, and the survivors ran away. Collected about 60 caged goblins and trolls. Had previously flooded half a level with magma getting said forges running, so put all them just beside the one magma-retaining floodgate.
Built the walls about 6 levels high, and was building it higher (for safety) when a large force of darkness descended from the other side of the volcano. Turned out the highest level of the walls was actually touching a ramp to allow easy access. Three times I scumsaved, and tried to hurry up the building of this highest level, and each time, the dwarves were slaughtered. Forth time I realised that if I didn't try building higher, and didn't build the next level of stairs, they couldn't get into my fortress... Facepalm moment when I realised that in the previous three attempts I started off by hurrying to build a stairway around my traps.
That attempt went better - except when confronted by several squads of elite crossbowgoblins, I thought dwarves would stay safe behind four levels of wall. No - they all climb up to the top level and get shot, until I manage to remove the lowest stair up the walls, leaving most of the population somewhere in the walls.
I decided to open the front drawbridge and let the fortress be slaughtered. Mayhem as I put every lever on "pull" and it looked like it was all over, until a jeweller got enraged and chased four trolls around the map, killing them all, and the two surviving members of militia seemingly chased half the goblins off the map.
But by this time I'd opened the magma floodgate (lower levels, central staircase leading into caverns). Goblins in metals traps roasted. Hydra in wooden trap didn't, and escaped, killing my newly legendary wrestler. Meanwhile lava hitting the cavern water slowed the FPS right down.
86 dwarves started that invasion, 34 survived, but just as I built 52 coffins and tombs, I got 27 migrants, including some useful skills, and all looked good, when the game crashed. I'm not going through that trauma again...