The Tacitus enclosure is the key building in the Forgotten Fortress which contains the Tacitus. Destroying the enclosure and acquiring the Tacitius is the final objective of the endgame.
The enclosure is located at the very end of the Forgotten Fortress, heavily guarded by max-level defenses such as particle cannon and busters.
The alliance that destroys the enclosure will bring down the entire Forgotten Fortress. The alliance will subsequently possess the Tacitus and its mysterious power, becoming the permanent dominant force on the world.