Tenshu (天守, 天主, 殿主, 殿守, also called tenshukaku, 天守閣) is an architectural typology found in Japanese castle complexes. They are easily identifiable as the highest tower within the castle. Common translations of tenshu include keep, main keep, or donjon.
Tenshu are characterized as typically timber-framed, having multiple stories, being seated on ishigaki (dry stone) foundations, and having individual floors delineated by surrounding tiled eaves. Further, tenshu are typically decorated with varying patterns of dormer gables (chidori-hafu), and are capped with hip-and-gabled roofs (irimoya-hafu) with shachihoko finials. Not all Japanese castles originally possessed tenshu (e.g. Sendai), many well-known castles have lost their tenshu (e.g. Nijō, Edo), many have had the tenshu rebuilt on multiple occasions (e.g. Nagoya, Osaka).
While both the term tenshu and the emergence of tenshu as a distinct architectural typology occurred in the 1560s and 1570s, the early relationship between the etymology and typology are not well understood.