On the first level of my first dungeon one of the first bits the players encountered was a room with a big lever on the wall. Across the corridor from it was a corridor with lots of small rooms on both sides.

A trap could be detected at the start of that second corridor (*big* trapdoor extending the width of the corridor)

They made several trips before someone moved the lever down. And found out that doing so removed the "safety" on the trap door.

Basically the room with the lever hand been a guard room, and the corridor with the trapdoor and small rooms had been the cells for prisoners.

The trapdoor was an extra safety measure in case a prisoner managed to get the door to his cell open :-)