The Lost Tombs of Duat

The Lost Tombs of Duat is the 4th area in the game. It appears to be taking place in some pyramid. Here you can find enemies such as Bone Nibblers and Mummies. The boss of this area is Basinjin.

There's also a secret warp zone in this area. It's on the level 4-4. To get there, you have to break the wall to the left of the first columns (be careful not to fall in the lava). Just follow the tunnel and you're in the warp zone.