Level 1 Access: 29656

Level 2 Access: 43126

Level 3 Access: 56517

The last database entry is your reward for beating Dark Labyrinth without dying, but it's just a thank you page and a list of cheats. If you really want it, press T to open the cheats menu and then type YOUSHALLPASS to instantly open the exit door.

The Level 6 entry for what would be E3M6 isn't unlockable and only exists for flavor.