The usual: about the time I think "Hey, I should get a proper military force in order" I get ambushed by goblins and FUN ensues.
I had even constructed the site for maximum defensiveness by building in a series of small mountains, linking them with bridges, and then de-ramping the bases so there was only one ground-level entrance for the entire complex. Unfortunately, I still had a lot of civvies wandering the map, including some of my militia, who were then cut down before they could reach the barracks to arm themselves.
The trapped entryway killed at least one and maimed three others, but it wasn't enough.
I think the next fort is going to be a tundra/glacier/desert fort so there's absolutely no reason for anyone to be on the surface.
EDIT: Well. That was....interesting. Next attempt was in a haunted hills region with a volcano on a far northern island. Only gobbos as neighbors. The landscape was covered in evil grass, all the wildlife was zombies, and it more or less constantly rained repulsive fith. My party got ripped to pieces by zombie wild boars because they were too busy puking their guts out and trying to crawl to the nearest pool of stagnant water to clean themselves. I think the whole game went less than three days.
DOUBLE-EDIT:
Super Mario Bros.
No, not really. DWARF FORTRESS, of course.