In 1491, the city was home to the Spanish Assassins' local guild. That same year, the Assassins were purged by the Spanish Inquisition; however, they would be rescued soon after thanks to the intervention of Italian Assassin Ezio Auditore da Firenze. By 1503, the Assassins had reestablished Barcelona as a base of operations.
In 1491, Barcelona housed an Assassin's guild that, like its contemporary counterpart in Venice, disguised itself as a mere thieves' guild. It was hidden near the center of the city in one of the tallest buildings and was accessible by a hatch on its roof.
That same year, Rodrigo Borgia, Grand Master of the Templars, passed information concerning the local Assassins to the Spanish Inquisition. The Inquisition, under the control of Grand Inquisitor Tomás de Torquemada, were grateful for Rodrigo's aid in purging the city of what they thought to be mere atheistic heretics and charged the prosecutor Gaspar Martínez with this operation.
News of this purge reached the ears of other Assassins abroad, including the Italian Assassin Ezio Auditore da Firenze and Raphael Sánchez, who both traveled to Barcelona to rescue their comrades. By the time they arrived, however, the Inquisition had already discovered the thieves' guild and raided it.
Ezio Auditore subsequently assassinated Gaspar and saved an Assassin that was about to be burned at the stake right afterwards. With the rescue of the surviving Barcelonan Assassins, Ezio and Raphael moved on to do the same in Zaragoza.
Assassination of Queen Isabella
Around 1503, during the Liberation of Rome, Ezio sent a team of his apprentices to Barcelona to determine the extent of Templar influence on Queen Isabella I of Castile as the death of Luis de Santángel, a member of their order with ties to the queen, had deprived them of any more intel.
The apprentices located the room of Luis de Santángel and discovered his journal, which revealed that he had resolved to slowly poison the queen before he died. The Assassins had not sought her death previously, even saving her life from the Templars in the aftermath of the Granada War, and they were unsure whether Luis was acting out of revenge for the family he lost to the Spanish Inquisition or to combat Templar power.
After spending much time investigating her and watching her actions, they began to notice that many religious figures had come to dominate her. They also intercepted several Borgia letters which revealed that Pope Alexander VI, Grand Master of the Templars, was exploiting her religious fanaticism to intimidate her into absolute compliance. With the threat of Isabella spreading the Spanish Inquisition to Portugal, the Assassins resolved to finish Luis's assassination plot, using one of his old accomplices to resume the administration of the poison. Isabella passed away in 1504 at Medina del Campo as a result.
Spanish Civil War
During the Spanish Civil War, an Assassin cell led by Ignacio Cardona was based in Barcelona in May 1937. They were joined by Norbert Clarke, an Assassin sent by by the British Brotherhood as backup. Norbert attempted to manipulate Ignacio into using a Piece of Eden, the Koh-i-Noor to 'stop' the conflict in the city. Ignacio managed to resist the artifact's control and dropped it to the floor before passing out. He was later taken away by the Black Cross, Albert Bolden who was observing on the rooftops nearby.