Yagura

The Japanese term yagura literally means a storehouse (kura) for arrows (ya), but yagura were used to store all manner of weapons, food and supplies. Yagura were not just storehouses, but vital defensive platforms and lookout towers that were placed at strategic locations throughout the castle. Most corners of a bailey had a yagura, often called a sumi yagura (corner yagura). These corners provided a wide field of view and two sides for flanking fire. The corner yagura also protected the vulnerable corners of stone walls which were much easier to climb than the straight sheer sides.

Naming:

You can sometimes guess what was stored in a yagura or its purpose from the name: Teppo Yagura (guns), Hata Yagura (flags), Yari Yagura (spears), Shio Yagura (salt), among many others. There were also other special function yagura such as Taiko Yagura which kept a drum to sound out the time, Tsukimi Yagura for viewing the moon, and Ido Yagura for housing a well. Sumi Yagura (corner yagura) often had special naming conventions too. They were named according to the 12 signs of the zodiac as placed on the face of a clock. So with north at 12 o'clock a northwest yagura was called Ushitora Yagura, a southeast yagura was Tatsumi Yagura, southwest was Hitsujisaru Yagura and Northwest was Inui Yagura.

Structure:

Yagura were mostly built in the sotogata style, with the same characteristics of many main keep. Sotogata have a square base with each level a size smaller than the lower one. There are few extant yagura in the borogata style, but some examples are: Utoyagura (Kumamoto Castle), Nohara Yagura (Iyo Matsuyama Castle) and the Fushimi Yagura of Fukuyama Castle. See the main keep page for more details about these structure types.
Display at Utsunomiya Castle

Seiroyagura (井楼櫓)

Seiroyagura is one of the earlier forms of yagura. It is a simple structure made from wooden beams in a square. They were used as watchtowers in earlier castles and were erected on the battlefield to provide commanders with a better view.
Sakasai Castle Arato Castle Takane Castle

3 level yagura (三重櫓)

main keeps are basically really big yagura that evolved to 3 or more levels. Especially, in eastern Japan many castles built a large 3 level yagura in place of a main keep. Such main keep were also called gosankai yagura. The main keep of Hirosaki Castle and Marugame Castle are actually 3 level yagura. At Edo Castle, the 3 level Fujimi Yagura was used in place of the main keep after it burned down too.
Typical 3 level yagura
Edo Castle Takamatsu Castle Fukuyama Castle Nagoya Castle Hikone Castle Hirosaki Castle
Gosankai Yagura
Hirosaki Castle Kakegawa Castle Shiroishi Castle Shirakawa Castle

2 Level Yagura (二重櫓)

2 level yagura is the most typical yagura type and there are many surviving examples throughout the country. You see lots of unique shapes and uses for these yagura.
Edo Castle Nijo Castle Nagoya Castle Iyo Matsuyama Castle

1 level yagura

Hirayagura (平櫓)

This is the simplest yagura type. It's too short to be used as a lookout tower and not usually used as a sumiyagura. Hirayagura is most frequently used on the edge of an outer bailey like a sannomaru bailey that has no dirt walls. It is also frequently used as a connecting yagura to join gates, larger yagura or main keep.
Hirayagura
Sakasai Castle Matsumoto Castle Matsuyama Castle

Tamonyagura (多門櫓)

Tamonyagura is a hirayagura that has been stretched out. It can be used in place of dirt walls atop moats and embankments. Tamonyagura are often found connected to corner sumiyagura or main keep. They can be used to store goods, but also function as a very strong defensive platform.
Tamonyagura
Hikone Castle Iyo Matsuyama Castle Fukuoka Castle Himeji Castle