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Tenshu (天守, 殿主, 殿守 also called tenshukaku?, 天守閣) is the central tower or main keep of a Japanese castle. It is the most defining feature of the Japanese castle. Although previous castles usually had a defensible structure for residence and/or administration, the architecturally distinct tenshu was first constructed during the late Sengoku period, or Azuchi–Momoyama period (1573–1603). Azuchi Castle, built in 1576 by Oda Nobunaga, was Japan's first castle to have a tenshu.