Castle in Harpy Hunting Grounds (The Witcher 3: Wild Hunt)

There is a castle. In Velen. Destroyed. Marked with a question mark. Some creature flies above him, a basilisk or a wyvern. But the bridge is raised, and the castle is on the rock. I jumped down into the sea to the eternal respawn of drowners, swam around, dived, but did not understand how to get there. This is one of the question marks about hidden treasures, where not a dot is indicated, but an area with a white circle. And yes, this is not a plot (the plot has already passed).