Note: I could not find a way to have sound effects during the game.

Boom is a first-person shooter published by Hungarian company 576 Kbyte. The game was only released in Hungarian. This video shows an English fan-translation.

While the game is quite obviously inspired by Doom, it does not try to realize the free movement of it on the C64. Instead, maps are made of squares and you move field-wise with 90-degree turns, just like in dungeon-crawlers such as Dungeon Master of Eye of the Beholder. However, you don't simply shoot in the direction you're facing. Instead, you move a crosshair over the screen for aiming.
The goal in each level is to reach the exit. You need to find keys in order to open locked doors. There are three weapons: a revolver, a shotgun and a machine gun. You start off with the revolver, but no ammo. The other two weapons need to be found. Ammo can be found in each level and is also sometimes dropped by killed enemies. There are also medpaks to regain health. A small map of your immediate surroundings is always displayed on the screen. You can save and load any time in the game.