In (almost) every fort, there is THE LEVER.
THE LEVER is connected to every bridge, floodgate, mechanism, and anything designed to keep things in or out that is not a door. THE LEVER also has a few other things that may happen as a result of being pulled. Sometimes, the booze stockpile gets dropped into the caverns.
Sometimes, the zoo and arena get released.
Every now and then, an innocent hole in the middle of the dining room gets turned into a magma fountain. Occasionally, a pump stack turns on and the waterfall becomes too fast to drain. There are times when only source of water becomes mixed with forgotten beast extracts, while the dining room closes, emergency tantrum weapon stockpiles open, and outdoor weapon caches open up for everyone stuck outside to go on a grand, suicidal charge into the face of the enemy become personalized organic ammunition storage.
Only once have I ever attempted to cast the entire inside of my fort in obsidian. It didn't work, but the resulting pattern of lava | obsidian with pockets of lava, water, and random corpses | water was rather aesthetically pleasing.
It's more of an "up yours" lever on the chance that the fort were to fall. If the fort is to go down, it will go down in as dwarvenly of a way as possible, take as many invaders with it as possible, and make it impossible for anyone but dwarves to reclaim.