Horde: The Citadel is a sequel to the fantasy real-time strategy game, Horde: The Northern Wind. Continuing the story of the land of Semirechye, The Citadel further explores the conflict between the Semireks and the invading nomads lead by their khan Teimur. In The Northern Wind, Semirek leaders have been able to drive Teimur's forces out of Lower Urgal, however the khan had been able to ally with hermit wizards and fortify his lands. The only path to his main citadel, Vorezh, lies through the Land of Shadows no man dares to tread, and the Semireks will need to secure an alliance with the dreaded dragons to pass safely into Teimur's domain.

The Citadel essentially uses the same engine as its predecessor, retaining all of the original game mechanics but also adds new content in the form of units, structures, items and terrain types. The game runs in higher resolution as The Northern Wind, but the sprites are not scaled up so a larger portion of the battlefield is visible. Gameplay remains mostly the same, with the player having to explore each of the nine maps in order to solve the main quest, find necessary items, defeat or negotiate with opponents. The maps are also larger, and each usually has two neutral Semirek settlements for the player to interact with, while in the previous game, only one settlement was initially present on most maps.