Shipwreck Island (Also known as Sleuthwing Island) is an island near Club Penguin, and the birds have a castle there, near Greenshore. Shipwreck Castle is feared by everyone, except the Birds. Shipwreck Island has a low population and is sort of isolated.
Matrix made a complete map of Shipwreck Island and all of it's roads, geographical features, and cities. Here it is: