Kassandra is associated with Sparta as a mercenary in Assassin's Creed Odyssey.
Kassandra is one of the game's two selectable protagonists, and the player can also choose Alexios instead. Both are connected to a Spartan family and become wandering misthioi, or mercenaries, during the Peloponnesian War.
Odyssey is set in ancient Greece in the fifth century BCE, with the main historical conflict beginning in 431 BCE. The game visits both Spartan and Athenian territories, so Sparta identifies Kassandra's heritage rather than every place where she works.
The answer is not Athens, even though Athens is a major location and political power in the game. The story presents the war as a conflict between the Spartan-led Peloponnesian League and the Athenian-led Delian League.