The Den

The Den was an Assassin hideout located in Alexandria, Egypt beneath the Great Library.

Originally belonging to Apollodorus, a loyal friend and follower of Cleopatra, he gave it to Aya, who was being pursued by Gennadios, the Phylakitai of the city after she assassinated Actaeon and Ktesos in 48 BCE. Aya sought refuge in the hideout until her husband Bayek was able to assassinate Gennadios, allowing her to roam freely in Alexandria again.

When the Order of the Ancients manipulated Julius Caesar to spare Lucius Septimius and Cleopatra to release Aya and Bayek from her services, they formed the Hidden Ones and established the Den as their temporary base of operations.

Reference

 * Assassin's Creed: Origins