Citadelum is an indie simulation and strategy game. Players build a settlement by gathering resources, managing population needs through farming and animal husbandry, and constructing infrastructure like aqueducts. Citizens can be assigned roles as artisans or farmers, or trained into military units for defense against barbarian attacks.

The game expands beyond city building with world map exploration and tactical auto-battles featuring Roman legions. Players can establish trade routes to acquire exotic resources and complete missions for the Senate. A core mechanic involves interacting with Roman Gods through offerings and temples; gaining their favor can grant boons, but incurring their wrath brings punishment. This interaction adds a non-linear, replayable narrative element, combining city building, strategic expansion, and divine intervention.