In Assassin's Creed Odyssey, Kassandra's birthplace is Sparta.
Kassandra is born in Sparta and is raised in the city's warrior culture before the events that separate her from her family. Her Spartan origin remains important throughout the story, even as her mercenary career takes her across the Aegean world during the Peloponnesian War.
Athens is a major setting in Odyssey, but it is not Kassandra's birthplace. The game also includes Kephallonia, Megaris, Phokis, Corinthia, and many other regions. Odyssey is set in 431 BCE, at the beginning of the Peloponnesian War between Athens and Sparta. Players can choose Kassandra or Alexios as the playable character, but the canonical protagonist in later series material is Kassandra.