Not going to do this, don't have a comp up to the task, or time, but I WILL share my plan that I would do if I would:
Build a castle on a giant ice plain, with a thriving metropolis city in a hollowed out cavern underneath. The lever sits alone on a distant cliff that can only be reached by a narrow bridge from a narrow tower. The dwarf consigned to pull the lever is ejected from the fort, and the entrance forever sealed. He enters the bottom floor of the tower, and exits out the other side to cross the bridge...
You have two options here, to use that dwarf or return with an adventurer.
Either way, it's basically the same: He pulls the lever... and the narrow bridge drops out, dropping in relatively safely to a lower level as the doors to the tower far above seal. This is the entrance to the labrynth, a maze full of various captured beasts and windy tunnels, all of which were released when the lever was pulled. Aside from this, the fort runs along safely. The dwarf tries to navigate the labrynth and return to (/loot) the fort in time.
Just as he breaks through the labrynth, he reaches a small cliff in the metropolis cavern that lets him see what is about to happen, though he's spent an entire season fighting his way through. A stairway leads down to the fortress, though it is long and winding, circling the entire cavern before touching down...
Aboveground, the season changes to spring... and the ice that was holding up the castle thaws. No longer being supported by the stairway that connected it to the metropolis, the fortress of nobility crashes through the ice, through the water, and then through the ceiling of the metropolis. By itself, it kills everyone in the castle and leaves much of the city unharmed... but the hole in the ceiling allows the lake to flood in, flooding the cavern. The dwarf who came via the labrynth turns and flees, accompanied by any dwarves who make it up the long staircase in time, fighting his way BACK through the labrynth to escape the flood - but the labrynth has changed! Paths have opened and closed as the floodwater triggers several pressure plates, and now he is running out of time. Reaching his starting point (if he's lucky), he finds a stairway has been opened up that allows him to the first floor of the tower - but the door was sealed when the lever first pulled, and there is nowhere to go but up. At the top is another lever, this one blue. Pulling it opens a separate, secret door on the bottom floor, but releases the captured megabeast that was stored halfway up the tower. The hallways are narrow enough the dwarf must fight past it to escape.
Assuming he does all this successfully, just outside of the door is a pressure plate that drops an a 5x5x5 block of stone on him the moment it is stepped on.
The end!