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Constantinople Assassins Guild
The Assassin's Guild in Constantinople was created by Niccolò Polo and Maffeo Polo by the request of the Grand Master of the Assassin Order, Altaïr Ibn-La'Ahad, in order to hide his Codex and his Seals subsequent to the Fall of Masyaf to Mongolian forces.
Source:
Assassin's Creed: The Secret Crusade