Tenshu

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.

The first known use of the term “tenshu” can be found in Yoshida Kanemi’s journal, Kanemi kyōki, in the entry for the 24th day, 12th month of Genki 3 (27/1/1573):

「明智為見廻下向坂本…城中天主作事以下悉披見也、驚目了… 」

“Left the capital with Akechi [Mitsuhide] for Sakamoto for the purpose of a survey…in the center of [Sakamoto] castle, all saw that a tenshu (天主) was being built and were surprised…”

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Wikipedia