XCOM: Enemy Unknown is a turn-based tactical strategy game that incorporates role-playing elements and it is a reimagined remake of X-COM: UFO Defense. The game is set in the near future when Earth is invaded by aliens. The player is put at the head of the elite multinational military organization XCOM (Extraterrestrial Combat Unit) tasked to defend Earth against the invaders as regularly military actions remain without result. This is done through a series of missions where the player commands a small troop. Between missions the larger management aspect is tackled through research and development to create new technologies and weapons using captured aliens and technology recovered from the missions. That way, the underground HQ is expanded to have a larger reach with new structures and the player is also responsible for the management of finances and monitoring alien activity. Players also need to keep panic under control. If a certain region of country is left unattended for too long, the local government will cut back its contributions to the XCOM funding, slowing down the overall progress. There are also often cut-scenes that further an overall story.
For missions a squad of four to six soldiers is selected from a pool of about twenty. Missions are set in small areas usually centered around a specific building. Missions are shown from a 3D isometric-style, floating view and a fog of war shrouds most of the environment until the soldiers uncover it with their line of sight. Solid walls often become invisible while browsing to provide a full view of the environment. After guiding soldiers and determining a plan of attack, it is possible to switch to a much closer tactical view to decide how to attack. The action is done in turns and percentages are used based on the soldiers’ abilities and positions to determine how successful actions are. Terrains are often fully destructible leading to situations where new cover needs to be sought or the strategy needs to be altered after a part of the environment has been blown up. That way both active camouflage and suppressive fire can be used to maneuver around enemies. Soldiers are promoted to a certain class (Assault, Heavy, Sniper or Support) based on their actions in their first mission. Promotions open up access to special support abilities with additional weapons. Soldiers can be fully customized in appearance, abilities (also healing) and loadout. Next to regular weapons some aliens such as the Thin Man can poison soldiers from a distance and later on aliens are introduced with psionic abilities. Through salvaging and research, these also become available to the player. Soldiers that return from the missions need time to heal and cannot be assigned to a new missions right away with full health and abilities, so multiple squads need to be managed.
For the 70 missions there are ones that are pre-built and are identical for every player. The others (the majority) are randomly generated based on the player’s overall progress. Rather than completely randomizing everything, maps are selected from a pool of 80 with random enemy placement. Usually a mission follows the crash landing of aliens and there are always three countries asking for help. The player can only choose one and that influences the relationship and funding. A decision can be made based on the amount of panic in the region or the reward offered (usually new scientists or engineers). The money is also spent on launching satellites to track alien activity through a holographic view of the Earth called Geoscape, but a lot of them are needed as they only have a limited reach. At the same time satellites are often attacked, so interceptors need to be constructed to defend them. Satellites can only be launched from special silos and cost a lot of money. That way, balanced management is the only way to keep everything under control while a steady growth need to be maintained.
The campaign can be played in Ironman mode where the game cannot be saved during missions. In one-on-one multiplayer games can be played with a mix of humans and aliens in a single squad, which is not possible in the single-player campaign. Elements from the original game not present are the inventory system, the time unit system and the grid map that is always visible. Troops also are deployed right away and do not need to disembark from the transport.