Article: The Germans developed massive reinforced concrete blockhouses, some more than six stories high, which were known as Hochbunker "High Bunkers" or "Flaktürme" flak towers, on which they placed anti-aircraft artillery. Those in cities attacked by the Allied land forces became fortresses. Several in Berlin were some of the last buildings to fall to the Soviets during the Battle of Berlin in 1945. The British built structures such as the Maunsell Forts in the North Sea, the Thames Estuary and other tidal areas upon which they based guns. After the war most were left to rot. Some were outside territorial waters, and had a second life in the 1960s as platforms for pirate radio stations.

Now answer this question: What structure did the British build in the North Sea?
Maunsell Forts