Keys are artifacts seen in various video games. While most keys follow the Super Mario Bros. 2 or Super Mario World key format of carrying them in the characters hands, some can be stored separately in the inventory and are used automatically when needed. These are represented by icons that appear with the player's display at the top of the screen.
Keys are both optional and essential in this game. Sometimes, they are used for secret exits in many levels, but they are also used to unlock gates in order to pass through. Other levels have mini-bosses such as Big Eyes in Guts Man Quarry and the Death Chompers in the Corpse of the Behemoth guarding keys to unlock gates in order to pass through and get to the boss. Phantos are usually guarding keys too, so be careful!