Pyramid

How's this for nefarious? The bottom layer, in keeping with what I hazily remember of ancient Egyptian tradition, has the large obvious entrance - which of course doesn't connect to the actual tomb, to avoid plundering.

The hallways should twist and turn enough to confuse any adventuring party, especially if you play it old school and don't give your characters a map. I've found that just describing the intersections as the characters see them really helps enhance the labyrinthine quality of a maze.

Traps and fake doors should be abundant in this dungeon, and all other manner of horrible surprises - tomb beetles and statuesque guardians come to mind, and coming up with novel curses is always entertaining. You can even have a trap drop unlucky explorers from the second level to the first, effectively trapping them there for all time. Throw in some moldering remains of previous victims to really sell it.

Even if the tomb robbers make it to the actual tomb, they'll have to contend with not only the enraged mummy whom they are disturbing, but also the honor guard keeping watch in hidden rooms.