Four's Fortress is a location in the Land of Mountains. It is a citadel defended by the Garrison and Cliffside Path.
Five travels to this location in search of the sea treasure. She finds a Giant Crab off the coast of the citadel and, with Gabriella's help, kills and eats it. Three also comes to the citadel with Gabriella in order to kill her old experiment Cerberi.
Following false information, Zero arrives at the citadel in search of Four only to find that she is not there. Upset by this, Zero kills every soldier and monster in the fortress.
In Branch D, Zero finds Four at the citadel, completely under the flower's control. Four summons her dragon, Zophiel, to fight her sister. However, her dragon falls, leaving her to fight off Zero herself. Zero ultimately kills her and has Mikhail devour her corpse.