King Gradlon (Gralon in Breton), ruled on Ys, a city built on land reclaimed from the sea, and some times described as rich in commerce and the arts, with Gradlon’s palace being made of marble, cedar and gold. In some versions, Gradlon built the city upon the request of his daughter Dahut, who loved the sea. To protect Ys from inundation, a dike was built with a gate that was opened for ships during low tide. The one key that opened the gate was held by the king. […]
Dahut received the key from him and its misuse led to catastrophe. […]
Other versions of the legend tell that Ys was founded more than 2,000 years before Gradlon’s reign in a then-dry location off the current coast of the Bay of Douarnenez, but the Breton coast had slowly given way to the sea so that Ys was under it at each high tide when Gradlon’s reign began.
Source: Ys – Wikipedia