Although most coastal fortifications are on the coast, not all are. Instead, some are off the coast on islands, artificial islands, or are specially built structures. All the sea forts share the characteristic that at least at high tide they are completely surrounded by water. Some, such as Fort Denison or Fort Sumter, are actually in harbours, but most are off the coast. Again, some, such as for example Bréhon Tower, completely occupy small islands, although others, such as for example Flakfortet and Pampus, are on artificial islands built up on shoals. Fort Louvois is on a built-up island, 400 meters from the shore, and connected to it by a causeway that high tide completely submerses. The most elaborate sea fort is Murud-Janjira, which is so extensive that one might truly call it a sea