Citadel is a platform game with a focus on puzzle solving. Gameplay involves exploring the citadel in search for the crystals while collecting items and avoiding enemies. The game does not have any instant deaths. Instead the player has an energy level that decreases when he comes into contact with enemies or hazards. However, if the player loses too much energy in a room he gets teleported back to the start of it. Energy is also lost continuously due to radiation so the player cannot stand around too long. The energy can be replenished by collecting bottles and solving puzzles. The game is low on combat as most enemies cannot be killed. The monks are an exception as they can be killed by shooting spells in their faces. The puzzle solving involves picking up objects and using them at the right location. The player can carry two objects at the same time. Examples of objects include keys to open doors, ice crystals that freezes water and trampolines that can be used to reach otherwise unreachable platforms.