In the first Assassin's Creed game, Jerusalem is the city containing the Assassin Bureau led by Malik.
The 2007 game places much of its historical story in the Holy Land during the Third Crusade. Its major cities are Jerusalem, Damascus, and Acre, while Masyaf serves as the Assassins' mountain stronghold. Each city contains an Assassin Bureau that helps Altaïr prepare investigations and missions.
Jerusalem is especially important in the game's opening sequence. Altaïr, Malik, and Kadar travel there to recover an ancient artifact from Solomon's Temple. The mission goes wrong, and Malik later becomes the bureau leader after returning to Masyaf. A common mix-up is choosing Masyaf: it is the order's headquarters, but it is not the city bureau identified in this question.
The game uses historical locations while combining them with fictional characters and a science-fiction framing story. Its Jerusalem is therefore a dramatized game setting rather than a complete historical reconstruction.