While the game is presented through protagonist Desmond Miles, the bulk of the game is played as Desmond experiences the memories of his ancestors through an advanced device called the Animus. This provides a means of a diegetic interface, showing Desmond's ancestor's health, equipment, goals, and other features as part of the Animus interface. The Animus is based on the player controlling the assassin to maintain the synchronization between Desmond and his ancestor's memories. Performing actions that go against the Assassin's way or dying breaks the synchronization, effectively requiring the player to restart at a previous checkpoint. Furthermore, the player cannot explore outside areas that the assassin has not experienced yet. There are also abnormalities within the Animus from previous users of the device.
While playing as the Assassin characters, the games are generally presented as third-person in an open world, focusing on stealth and free-running. The games use a mission structure to follow the main story, generally assigning the player to complete an assassination of public figureheads or a covert mission. Alternatively, several side missions are available, such as mapping out the expansive cities from a high perch followed by performing a "leap of faith" into a haystack below, collecting treasures hidden across the cities, exploring ruins for relics, building a brotherhood of assassins to perform other tasks, or funding the rebuilding of a city through purchasing and upgrading of shops and other features. At times, the player is in direct control of Desmond, who by nature of the Animus use has learned Assassin techniques through the bleeding effect, as well as their genetic ability of Eagle Vision, which separates friend, foe and assassination targets by illuminating people in different colors. Through the Animus interface, the player can go back to retry any past mission already completed; for example, in Assassin's Creed: Brotherhood, the player achieves better synchronization results by performing the mission in a specific manner such as by only killing the mission's target.
The games use the concept of "active" versus "passive" moves, with "active" moves, such as running, climbing the sides of buildings, or jumping between rooftops, more likely to alert the attention of nearby guards. When the guards become alerted, the player must either fight them or break their line of sight and locate a hiding place, such as a haystack or a well, and wait until the guards' alert is reduced. The combat system allows for a number of unique weapons, armor, and moves, including the use of a hidden blade set in a bracer on the Assassin's arm, and which also can be used to quietly assassinate targets.
