The great fortress of Osteral, "Foldvessels" is much like Helm's Deep, almost impenetrable defenses (for me it's rows upon rows of weapon traps at the entrance completely stocked with large serrated discs, I refer to it as The Goblin Blender) save one thing, one sewer opening. Granted mine is more of a tunnel under a lake that supplies the wells of the fortress with water.
It didn't occur to me that this spot was a weakness until some alligators outside suddenly showed up in the grand meeting hall. They had climbed out of the well and a few were struck down before soldiers could come and dispatch of these vile creatures. Some engravers felt this something worth carving into the walls, so as to warn all future dwarves, but I thought my troubles were over as long as I kept an eye out for any more alligators.
(I would love to see that engraving of the alligator poking it's head out of a well and attacking a dwarf)
But just like Helm's Deep, invading goblins have been exploiting this weakness. Nearly every units during any given siege are mounted atop Giant Toads. A few more dwarves died within the grand meeting hall when I first learned of the Giant Toads and their riders also being able to climb through the well.
Now I'm trying to decide whether I should wall off the pond that provides water for the fortress or if I should endeavor to put a grate up somewhere to block invaders and alligators out.