It is a Norman type castle with a keep, built at the end of the 12th century to fill a gap in the city's defences after a Manx raiding party failed to loot the city but instead stole a number of ships. It was blown up during the Great Rebellion by parliamentary troops, but was refortified during the Napoleonic Wars. The demolished masonry slid down into the ocean, blocking the access for most boats into the Harbour. Because of this the Harbour has since been turned into a marina, which has become well-known for yacht traffic and regattas.