The Backdoor is the fifth level in Duke: Nuclear Winter.
This level appears to be an underground facility; the exact purpose of it is unknown; Duke must make his way through the facility to get to Christmas Village . This level mostly consists of going through a network of underground tunnels and sewers with the odd underwater combat with Octabrains. Apart from the occasional Christmas tree this level doesn't really have anything to reflect the Christmas atmosphere and there isn't really anything notable in the level.
secret 1 - after you blow the trip lasers, jump into the right side wall for atomic health