Duke Nukem 3D: 20th Anniversary World Tour

You can do this on any of the 7 secret levels in the game, but for this guide, we are going to unlock this on the level ‘Tier Drops’ accessed from the level ‘Movie Set’ in Episode 3.

From the main menu, select Single Player > New Game > Shrapnel City > Movie Set and select your difficulty. When the level starts, activate the cheat menu. Turn ‘God Mode’ on and ‘No Clip’ off. It might help to turn ‘Show Map’ on as well in case you need to backtrack.

Walk through the door on the left (under the Stage 17A sign) and turn right to go through the stage door. Walk straight ahead until you reach the moon set and walk through the tech wall on the right. This will reveal a secret Nuke Button. Punch it to end the level which will take you to the secret level ‘Tier Drops.’

When this level starts, walk straight through the wall and you will be in this big room with an alien structure in front of you. Continue walking straight through the structure and you will eventually appear at the top (if you go too far, you will warp into a cavern surrounded by Octabrains. Just backtrack to the start and try again). Walk around to the left and there will be a door with a Battlelord face on it. Open the door and at the far end of the room is the Nuke Button. Punching it will end the level and unlock the achievement.

Thanks to asd asd for the video: