The Citadel seems to pay tribute to classic shooters, using mechanics and gameplay principles familiar to this genre. The plot centers on the confrontation between people and the so-called Sleeping God. Previously, this creation was literally sacred, but suddenly something went wrong, and people began to turn en masse into deadly warriors. Their only goal is to kill, and it doesn't matter who gets in the way. The main character, nicknamed the Martyr, did not succumb to the influence. She will have to get into the very heart of the rebellious mechanism and stop the process before it is too late.

The Citadel's story campaign is divided into episodes, the gameplay in each of which is extremely simple. You need to fight your way through crowds of enemies, discovering secret rooms and collecting keys to get to a new location. The arsenal of weapons is quite diverse and contains several unusual barrels. For example, there is a gadget available that causes a missile strike in an area, or a sniper rifle that breaks through walls. There are 14 weapons in total, each of which has an alternative firing mode. In addition, at some points you can resort to hand-to-hand combat, throwing the enemy away from you. Each stage ends with a battle with a boss, for each of which you should select a separate tactic. Another hardcore feature is the control of vital signs – health, air and hunger. The latter is especially important, since if the heroine gets hungry, she may simply die. Therefore, you should inspect all locations in search of supplies.