tenshudai 天守台

The independent raised foundation upon which the principal tower *tenshu 天守 was constructed. Usually approximately square shaped. The first tenshudai of the late Medieval period are thought to have been made of earth alone, but all those of the Edo period consisted of stone-faced earth walls *ishigaki 石垣. The incline of the tenshudai walls was relatively straight where built on solid ground, but concave ramparts were preferred where the soil was weak. This concave type was most common; the line of the curve is called *ōgi-no-kōbai 扇の勾配. 

Various methods of stone facing were used on the ramparts: (1) a facing of rough, natural stones *nozurazumi 野面積 as seen at Matsuyama-jō 松山城, Ehime Prefecture and Kōfu-jō 甲府城, Yamanashi Prefecture; (2) partially dressed stones *uchikomihagi 打込矧ぎ as at Himeji-jō 姫路城, Hyōgo Prefecture; (3) well dressed and carefully shaped stone set in a well ordered arrangement *kirikomihagi 切込矧ぎ as at Edo-jō 江戸城, Tokyo. Gaps between large stones were filled with irregularly shaped smaller stones kai-ishi 飼石.

The tenshudai was not necessarily solidly packed but sometimes had an excavated interior that might hide one or more basement floors not visible from the outside. Surviving examples demonstrate that some foundations were very large. For instance, the military records Shinchō kōki 信長公記 (ca. 1600) record that the stone-faced foundation of Azuchi-jō 安土城, Shiga Prefecture, was more than 24 m (12 *ken 間) high. In many castles today, the tenshudai is the only surviving part of the principal tower. For examples, see Osaka-jō 大坂城, Osaka, or Edo-jō.