Sparta is the ancient Greek region associated with the birthplace of Alexios and Kassandra in Assassin's Creed Odyssey.
The siblings are children of Myrrine and Nikolaos, and their family is tied to Sparta at the beginning of the story. The game's opening sequence takes place at the Battle of Thermopylae, where King Leonidas I leads Spartan forces against the Persian army. The later family narrative connects the protagonists to Spartan society and the wider Greek world.
Odyssey's map includes many regions and islands, including Attica, Lakonia, Phokis, Boeotia, and Crete. Sparta itself is located in Lakonia on the Peloponnese, and the game presents the city and surrounding region within its fictionalized fifth-century BCE setting.
The answer is Sparta rather than Athens, which is another major Greek center in the game. Athens is especially important to the political and philosophical story, but it is not the birthplace associated with the two selectable protagonists.