DunGen

DunGen:
Your goal from the beginning is to kill the boss. You can do it at any moment by building a room on the 0 key. However, it doesn't mean you should. 

Gather XP from enemies in the dungeon you build and prepare for the final showdown.

Mini Tutorial:

Building is done by pressing the E key by one of the doors, to select the type of room, press the corresponding number from 1 to 9. The boss room is assigned to the 0 key. To exit build mode, press Q.
Press spacebar to add rooms to your current deck. The rooms are marked with difficulty levels, so carefully assess your capabilities when building.

You gain XP by killing enemies and destroying chests found in the rooms. Each level gained halves final boss' health. 

Chests with powerful XP boosts are hidden on the edges of the world.

Explore and find them.

P.S. 

All assets were taken from Kenny's Crosshair Pack. And some others.