Alep

Alep was an assassin fortress during the Third Crusades. It was the place where Harash, the Assassin Order's second-in-command betrayed the assassins and was later killed by Altaïr Ibn-La'Ahad. Before the events of original Assassin's Creed, Al Mualim resided in this fortress until it nearly come into disarray. In 1190, Altair visited Alep twice.

Assassin's Creed: Altair's Chronicles

The first visit was during him returning from an assignment. He found the fortress and a small village nearby besieged by Templars under Lord Basilisk. After repelling the attack, Altair met Al Mualim, who gave him a new assignment concerning the Chalice.

During his second visit to the fortress, it had been already corrupted by the traitor Harash, who established his own guards around it. Templars had also made their way inside as well. Despite Altair managing to cleanse the fortress, Al Mualim and the Order had to switch their residence back to their home, Masyaf.

Assassin's Creed

While Alep never appears in the game, it is mentioned by an Assassin informant in Damascus, during the assassination attempt on Abu'l Nuqoud. The informant asks Altair about Adha and mentions their past adventures in Alep, probably mentioning the time when Alep was the Assassin's temporary headquarters.