Falling Portcullis
Generic Hazard
Some folk who build dungeons, such as mad wizards in search of new victims, have no intention of allowing their visitors to make an easy escape. A falling portcullis trap can be especially devious if it causes a portcullis to drop some distance away from the pressure plate that activates the trap. Although the trap is deep in the dungeon, the portcullis closes off the dungeon entrance, which is hundreds of feet away, meaning that adventurers don’t know they are trapped until they decide to head for the exit.
Source: Xanathar’s Guide to Everything p. 113