The 5,000 years old structures include the ruins of a castle, a triumphal arch, five temples, a sphinx and at least one large stadium, all of which are connected by roads and water channels and are partly shielded by what could be huge retaining walls. The legend of the giants in Yonaguni Island tells us that giants from a city across the sea often visited the Kavalan people of Yilan, who used the strength of these giants to help them build any kind of construction in exchange for food: https://humansarefree.com/2020/10/underwater-ruins-of-yonaguni-and-the-legend-of-the-giants.html