1. Rinkaku Style:

The rinkaku style has the hon maru (inner courtyard) in the center and the ni no maru (second bailey) and the san no maru (third bailey) arranged in concentric rings around it. The moats and the stone walls of such a castle had to be extremely long compared to the small area on which the hon maru was built. Shizuoka and Tanaka castles follow this style and Osaka is an approximation of it.

2. Renkaku style

The renkaku style has the hon maru in the centere with the ni no maru and the san no maru on either side. With this style it is necessary to provide extra protection for the more exposed hon maru. Mito and Sendai are examples of this style.

3. Hashigokaku style

This style can only apply to a yamashiro (castle built on a mountain) setting. The hon maru forms the apex of the castle while the ni no maru and the san no maru descend in steps like a staircase. As the Hon Maru is exposed on the one side, it needs to back onto a lake, river or cliff. Aizu-Wakamatsu is an example of this style. Inuyama has a dramatic cliff on the hon maru side.

In the case of a larger castle, the three maru would in turn be encircled by two or three outlying kuruwa (successive courtyards) reffered to as sotoguruwa (outer courts) or soguruwa (outer courts). Each of these successive areas was so arranged that any line of defence captured by an enemy could readily be recaptured from the area inside it. The ultimate result was a maze of interlocking walls and gates that would confuse an enemy and allow him to be observed for every inch of his way up to the keep. Himeji is an example.

(From a book called Japanese Castles 1540 - 1640 by Stephen Turnbull and illustrated by Peter Dennis. Osprey Publishing)