Tiber

The Tiber river (Italian: Tevere) was a river that ran through the city of Rome, which separated the Vaticano district from the rest of the city.

The Castel Sant'Angelo rested on the river bank, whilst Tiber Island, the only island upon the river, could be found further south.

Bridges
There were six bridges built over the river, allowing civilians to cross the river from various points in the city. Among them were the Ponte Sant'Angelo and the Ponte Sisto.

Only two of these bridges crossed the river to Tiber Island; both of which were marked with the Templar Cross. No other bridge in Rome had these symbols, yet Tiber Island housed the Assassins' headquarters.

Trivia

 * Ezio Auditore da Firenze considered dropping the Apple into the Tiber River after the confrontation in the Vatican in 1499.
 * There are hidden entrances into the Tiber Island headquarter submerged in the Tiber.
 * The iconic scene of the Assassin Ceremony ends with the newly-initiated Assassin performing a Leap of Faith into the river.
 * Cesare Borgia would often have the bodies of his enemies discarded into the river, including his brother Juan, and Francesco Troche.

Source

 * Assassin's Creed: Brotherhood