In Assassin's Creed Origins, Siwa is the game's main starting location.
The story begins in the Siwa Oasis, where Bayek of Siwa works as a Medjay. Bayek returns to the region after pursuing masked enemies and soon becomes involved in a wider conspiracy threatening Egypt. The oasis establishes his identity, relationships, and motivation before the journey expands to other parts of the country.
Siwa lies in Egypt's Western Desert, far from the densely populated Nile Valley. Its distinctive oasis landscape contrasts with later locations such as Alexandria, Memphis, and Cyrene. The isolated setting also helps introduce the game's exploration, combat, and investigation systems without immediately placing Bayek in Egypt's largest urban centers.
Alexandria is an especially common alternative because it is a major location and home to Cleopatra's court. It is not the opening base, however. Origins deliberately starts in Siwa before Bayek travels through a much broader representation of Ptolemaic Egypt.