yagura 櫓

A guard tower set atop the corners of a castle's stone wall and other strategic positions overlooking the surrounding area. Originally the term referred to a watch tower *seirō 井楼 constructed at a time of war. It had a simple wooden framework and no roof. Gradually it became a solid timber-frame structure with a tiled roof and thick walls aligned with the plane of the stone embankment and outer castle wall *hei 塀. 

Often it is connected to walls and other yagura, and looks like a connecting gallery. It is classified by location, function, and appearance. The yagura gate *yaguramon 櫓門 serves as a two-story gate in a barbican gate complex *masugata 桝形. The outward appearance varies: there are single and double-roofed towers, four-cornered towers, and triple-roofed structures. 

The origin of the term is said to be 矢倉 (arrow storehouse), and later some served as armories. Others functioned as storehouses for rice or salt. The idokuruwa yagura 井戸郭櫓 housed the all important well. A *tsukimi yagura 月見櫓 might have an open design for moon viewing. 

For specific examples and illustrations, see also some of the other different types of yagura: corner tower *sumiyagura 隅櫓; projecting tower or gallery deyagura 出櫓; many gallery tamon'yagura 多聞櫓 (see *tamon 多聞); and connecting gallery/tower *watari yagura 渡櫓.