What is "tenshu"

thumbnail|250px|right|Tenshu at Matsue Castle 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 between 1576 and 1579 by Oda Nobunaga, was Japan's first castle to have a tenshu.