Have a door with a magic mouth and a Glyph of Warding. The mouth would give a really really easy riddle, "speak the name of the beast which is there and not there". The party should come up with the answer "Displacer Beast", have the words "Displacer Beast" be the trigger for a glyph of warding. I'd also cast phantom trap on everything I could think of. Nothing makes a trap harder to detect then an entire room, where every object detects as a trap. The thief would have to attempt to disarm every item in the room before being sort of safe. Also every door or chest could be arcane locked with poison gas tubes built into the door. The party either will expend allot of resources on knock or dispel magic or they will be forced to break down the door. I'd also use the Magic Aura spell to make worthless items, preferably bulky items seam useful. Nothing like a few highly visible nonmagical 50 pound 6 foot long lead rods which radiate strong magic to limit the amount of goods that can be stolen. If you cast continual flame of the 50 pound rods they are sure to be stolen.