Tiber Island
Tiber Island (Italian: Isola Tiberina) is an island located on the Tiber River, 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
The building was initially owned by Fabio Orsini, but it was given to Niccolò out of Fabio's wish to gain freedom from the Borgia. 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 the Assassin recruits into the Order, and grant them the rank of Assassino. 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.