In Assassin's Creed Odyssey, the ancient Greek city associated with Myrrine's birth is Sparta.
Myrrine is a major figure in the game's family story and a member of the Spartan royal line. She is the daughter of King Leonidas I, the Spartan ruler whose stand at Thermopylae appears in the game's opening sequence. Myrrine later becomes the mother of the protagonist and is central to the search for the player's divided family.
Sparta is therefore important both as Myrrine's homeland and as part of the protagonist's heritage. The game is set during the Peloponnesian War, beginning in 431 BCE, and its world includes many regions and cities such as Athens, Corinth, Argos, and Thebes. Sparta and Athens are especially prominent because they lead opposing alliances in the war. The answer is Sparta, not Thermopylae: Thermopylae is the famous pass where Leonidas fought, while Sparta is the city-state connected to Myrrine's lineage.