The door to the dungeon is a trapdoor in the steps, very well-hidden (but able to be found easily by the party rogue), and opening downwards when triggered by a tripwire (also able to be found) on the step above. The PCs will think that this is a trap, and step over it (which they can do without too much difficulty). However, when the rogue tries to pick the lock on the door at the bottom of the steps, the steps become a ramp, which is Greased, and a Telekinesis spell ensures that the party start sliding. The door is an illusion, concealing a Gate to some nasty plane or other...
