The Isu built the city of Atlantis in Assassin's Creed Odyssey.
In Assassin's Creed lore, the Isu are an advanced precursor civilization that existed long before recorded human history. They created powerful artifacts, including Pieces of Eden, and influenced later human myths and religions. Odyssey's Atlantis content presents the city as an Isu realm rather than as an ordinary human settlement from classical Greek history.
The Fate of Atlantis downloadable content sends the protagonist through realms associated with Isu mythology and explores the civilization's technology, rulers, and relationship with humanity. The content connects the ancient storyline to the series' recurring themes of precursor artifacts and the origins of the human-Templar-Assassin conflict.
The historical Atlantis described by Plato is a philosophical and legendary subject, not an established archaeological city. Assassin's Creed adapts that legend into its fictional Isu mythology. Greeks, Romans, and Persians appear throughout the series, but they did not build the fictional Isu city in Odyssey's narrative.