Assassin bureau
There is an Assassin's Bureau in all three cities, it's basically a safe house for Assassin Instructors in the field and the Assassins Order. The Assassin Bureau's leader provides you with useful hints on where to obtain your information. As soon as you acquire enough information the leader will provide you with what appears to be one of the primary flight feathers of a white eagle, which is yet another nod towards the resemblance that Altaïr shares with the birds. Once you have collected all the required information on an assassination target, and it is possible to do so before entering the Bureau, the leader of the Bureau within that city, provides you with that same feather which is used to claim an assassination by running the Feather through the blood of the target. As proof of the assassination, Altaïr must return to the Bureau and present it as proof of his deed. The Assassin's Bureau will be closed when your current social status is exposed, to prevent guards from discovering the Bureau and also to not have you violate the Creed (never bring attention onto the Assassins Order). The Assassin Bureau is a place where they also make swords, blades and other useful equipment. Just before assassinations the Bureau is used to prepare. The entrance of the Assassin's Bureau is located on the roof probably to reduce the chances of outsiders wandering in off of curiosity. After all information pertaining to high ranked city officials resides within the Bureau.