noun
castle tower; keep; donjon
Specifically refers to the central, multi-storied tower of a Japanese castle, often used as a symbol of the castle.
姫路城の天守閣は国宝に指定されている。
The castle tower of Himeji Castle is designated a national treasure.
城 is the general word for 'castle', while 天守閣 refers specifically to the main tower within the castle complex.
Compound of 天守 (tenshu, 'castle tower') and 閣 (kaku, 'tall building'). The term 天守 itself may derive from 天主 (tenshu, 'Lord of Heaven'), reflecting the tower's symbolic and defensive prominence.