Siege of the Citadel 2nd Edition is a reboot of Mutant Chronicles: Siege of the Citadel and is set in the Mutant Chronicles universe with each player controlling one of five corporations: Imperial, Cybertronic, Mishima, Capitol, or Bauhaus. The players must prevent the Dark Legion and one of its vile leaders from conquering the solar system. Each successful mission brings news of monstrous foes and terrifying events as the heroes explore the modular Citadel. Promotion gives credits to access more powerful weaponry and abilities, while secret missions can cause chaos for the heroes!

The narrative-led design of Siege of the Citadel updates and balances the gameplay of the original release and adds new campaigns based on major events from the Mutant Chronicles universe and new creatures, such as the Praetorian Stalkers.

During each mission a different player will control the Dark Legion forces, while their fellow players attempt to fight their way through the citadel.

Players select one of five factions (each with a unique faction ability) with two figures representing a team. Characters have 5 wounds, but their team shares promotional points, which are now tracked in units of 10 and through rank cards as the heroes increase in experience. Teams have secret secondary missions which may be at odds with the rest of the teams in the game and doomtrooper cards can give tactical advantages or be discarded to take an extra action.

As players take it in turns to play the Dark Legion they get to control the vast Citadel of Alakhai on Venus, its defences and the terrifying creatures ready to do battle with humankind. Each turn they will summon reinforcements, play event cards to further challenge the doomtroopers and try to defeat their friends with the forces of the Dark Legion. If the Dark Legion player does well at the role, his doomtrooper team gains promotion points to advance their characters’ rank for when they play the next mission!