Coral Depths, magnificent leveling area and treasure trove. Unfortunately often rife with golders.
The key to the castle is dropped (and hidden) by the crab at the gate. Neptune sometimes drops the key for his throne and Maerdei always does.
Walk ( 6s, ne, 2s, se, 2e, s, 2sw, s, 2e, u, 3e, ne, 2e ) to get from DH to the room marked in blue on the first image.