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
Gosankai Yagura
Hirosaki Castle Kakegawa Castle Shiroishi Castle Shirakawa Castle

All Pictures...