It's not a trap door. It's the second door that leads into the Dark Forest that was sealed off. Remember in season 1, there were two active doors in that room? Any layout with the Jester's Court only has one active door.
My question, is why couldn't the temple gaurds sit or stand in the position of the wall paintings, and then seemingly "come to life" and capture the kids. They could exit through the doors in whatever room the contestant just came from or just hop the front. They also could have crouched down in front of the room behind hte ledge and popped up to capture the kids, too.
EDIT: Oh wait.... i'm thinking of the wrong door... carry on.