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.
Most of the new units introduced in The Citadel have the ability to use two attacks, melee and ranged, something which was not possible in the original game. Additionally, the player can produce air units - pigeons - to quickly scout the map. New monsters had been introduced as well, such as dragons, crocodiles, scorpions, were-bears and magic riders. The game re-uses portraits of non-player Semirek leaders from The Northern Wind, but with different names; key plot-related characters however have new portraits and, in some cases, new sprites to represent them on the map.
There is an option to import characters from The Northern Wind into The Citadel, so that all items and units that were present with the character when the first game was won will be carried over into the sequel.