Another idea that I had was to have a winter death trap, where the room would be filled with water and then the roof hatches opened to freeze the water. Would this work? And how deep does water need to be before it kills people when it freezes?
I wanted to do something like this, but I could never get it to work. As far as I can tell, a particular tile is either considered "open to the air" or "indoors" regardless of how many floors of stone are over it. I never did master using floodgates or drawbridges to affect freezing and judging by the discussions I've seen on here I don't think it's possible.
I did get a freezing room to work on some goblins, but what I had to do was use water to wash goblins down from an indoor area into a ditch, this ditch then led outside and into an open-air trench with steep sides. If the goblins attacked me in the summer, then they got to swim or drown in that trench until winter, at which point the water would freeze and they'd get encased in ice. I had to do all the "flushing" in a season other than winter, because otherwise only one tile of water would freeze and the water wouldn't flow into the far end of the trap.
Sorry, I haven't yet tested what depth of water it takes to freeze somebody. I heard 1 will work, but I haven't confirmed it yet.
And if we're still talking about your avatar, I like it. I get a sort of "Newgrounds" vibe off of it, whether that's good or bad.