My problem has been the game slowing down once it reaches a certain size.
For one of my fortresses I setup next to a river and then dug a moat. The goblins had to go through one area that I had left as an entrance which was also filled with traps. I think that one fell when a forgotten beast came up through one of the tunnels and started killing people.
One fortress I had to abandon when I dug into a underground lake from below (on accident)while building a second shaft for dwarves to use. I had somehow missed a underground layer completely in my attempt to dig down to find lava.
