Tiber Island
Tiber Island is an island located on the river Tiber, in central Rome. During the early 16th Century, the island served as the headquarters of the Order of Assassins in Rome, and as a base of operations for Ezio Auditore da Firenze's war against the Borgia. While Ezio went out to undermine the Borgia's rule over Rome, Niccolò Machiavelli stayed on the island, training new Assassin recruits sent to him by Ezio. On the in-game map, Tiber Island is marked by a white Assassin symbol on a black background.
Assassins' Guild
Externally, the guild is a very tall but largely unassuming building, which can be entered from either the roof, the street, or the river.
Function-wise, it is similar to the Villa Auditore in Assassin's Creed II, with a few additional features. Its rooms include an Armory that displays armor sets and weapons, a Painting gallery that showcases purchased paintings and the portraits of fallen targets, and a ceremonial hall that is used to induct new Assassins into the Order. A View Point on the roof is used for the initiation Leap of Faith performed by the recruits.
Any recruits of the Order can often be seen milling around or speaking to each other in the rooms of the guild when not out on missions.