Almost everyone should, by now, know how to construct many different kinds of elaborate death traps inescapable even by deadly building destroyers. It's not difficult, and it works as a fool-proof way to safe-guard your fortress. But how would you go about safe-guarding your fortress against intelligent invaders (for now, only adventurers fall under this category, but as Toady improves the AI we can hope for something more) ? Secret doors and time-released beasts all fall under this category. I've been able to do a series of gates which open only by someone who knows the correct combination of moves--one wrong move and it's game over.
What I'm attempting to do is create a list of possible traps that will work for this scenario, I'll add suggestions as we go along.
1) Combination Locks -- This includes doors, floodgates, etc which only open via a series of pulled levers or pressed plates. I've always done it with the levers releasing water into different chambers via hatches. This has an added bonus of allowing you to flood rooms due to wrong answers. You can do it as simple as you want, or more complex, if you want to get into random bit sequences.
2) Collapsing Rooms -- A ceiling held up by supports set to collapse at a trigger.
3) Timed Beast Release -- When the Invader enters your fortress, he has only so much time to leave before the beasts are released.
4) Death Maze -- Twisting passages filled with strange creatures, mysterious levers, and surprising pressure plates all designed to confuse the invader. With the right combination of bridges and pressure plates, the maze can change infinitely.
5) Ghosts??? -- Can ghosts be used as a trap?
6) Untested Secret Door -- I would think this could work by building a wall and collapsing something on top of it, or taking out its supports. Has anyone been able to do something like this?