The player is in a gigantic procedurally-generated maze, like a Nethack level with thousands of rooms. A bit of fun comes in exploring, discovering the different monsters, and collecting $ to raise your stats.
The combat interface is amazingly simple. You hold down the space bar and let go to launch an attack that hits everything in the room. Enemies don't take damage, but they either die or they don't depending on how long you held down the space bar.
Elegant simplicity is repeated in how the game uses colour. It looks monochrome, but the game's one colour changes from blue to black due to how far the player is from a safe area.
There are hidden items in the game, and all of them are passive-effect. Simplicity reigns.
There's more to the game, but it would spoil it if I told you. I will say that right when I thought I'd played it out, adapted to all the monsters and discovered everything interesting, the game decided to double all the monsters' speed, endurance, and fire rate. The core gameplay may be repetitive, but the game has a few ways of making itself interesting.