Under the German Empire, the islands became a major naval base, and during the First World War the civilian population was evacuated to the mainland. The island was fortified with concrete gun emplacements along its cliffs similar to the Rock of Gibraltar. Island defences included 364 mounted guns including 142 disappearing guns overlooking shipping channels defended with ten rows of naval mines. The first naval engagement of the war, the Battle of Heligoland Bight, was fought nearby in the first month of the war. The islanders returned in 1918, but during the Nazi era the naval base was reactivated. Lager Helgoland, the German labour camp on Alderney, was named after the island.