Where is volka castle on endless ocean blue world

there is a secret passageway in the well of the Triton place with the shipwrecks and sharks. you go inside and follow the tunnel. then your in a tower, and you swim up until you see a doorway on the side. you go in the doorway and you are in the main hallway. then you can go in any of the rooms that are branching off. use the multisensor to find buried treasures.