Alderney shares its prehistory with the other islands in the Bailiwick of Guernsey, becoming an island in the Neolithic period as the waters of the Channel rose. Formerly rich in dolmens, like the other Channel Islands, Alderney with its heritage of megaliths has suffered through the large-scale military constructions of the 19th century and also by the Germans during the World War II occupation, who left the remains at Les Pourciaux unrecognisable as dolmens. A cist survives near Fort Tourgis, and Longis Common has remains of an Iron Age site. There are traces of Roman occupation including a fort, built in the late 300s, at above the island's only natural harbour.