The large tower seen in many pictures of castles is the main keep. It is nestled in the most central courtyard among the maze of moats, walls, gates and courtyards that make up a castle. The main keep is not the castle itself. It is only one part of the castle. In fact, not all castles even had a main keep. Many early castles or mountaintop castles did not have one. When a main keep was burned down by fire or destroyed in an earthquake, it was sometimes not rebuilt. This is because during the relatively peaceful times of the Edo Period the main keep was less important than earlier times, the costs may have been too prohibitive or in some cases the Tokugawa government simply wouldn't give permission to a lord to rebuild the main keep. During the Edo Period, lords were required to get approval for any major improvements to their castles.
Main keeps were built on top of a high stone foundation or along the top of the stone wall on the edge of the bailey. Like all Japanese architecture, castles relied heavily on wooden construction. This made them much more vulnerable to fire and artillery than the stone keeps of European castles. The exterior walls of most main keeps, as well as other castle fortifications, were covered with thick layers of plaster for protection from fire and artillery. Even so, a great many castles have burned to the ground from fires caused by earthquakes, lightning or attack. The fear of fire was so prevalent that killer whale talismans (shachi) were mounted on top of all main keep and many other fortifications to protect them from fire.
Main keeps can be classified by two major methods, the architectural structure and the style of the main keep. It's difficult to explain some of the concepts with words or with the pictures I have, so I created some basic Google Sketchup diagrams that you can see for each subcategory below. These are intended to be simple visual aids and not perfect scale reproductions.
There are two architectural structure types for main keep: sotogata and borogata. It is usually fairly easy to distinguish between the two. Borogata has an irimoya gable (see the starred part of the picture below) and it looks like the lower floor(s) and the upper floor(s) are of different styles, with the upper floors being square. If not, it is sotogata.
|Hikone Castle||Osaka Castle (2 irimoya gables)|
Borogata literally means "lookout tower type." This is the older type of main keep built when the techniques for building stone foundations were less advanced. It is built on an irregular shaped stone foundation that is not perfectly square. An irimoya style gabled roof is used on the first or second floor to provide a roof for the lower levels of the main keep. A square shape is created in the middle of this roof on which the upper levels of the main keep are built. The upper levels are square and balanced. It should give the impression of a lookout tower mounted atop another building. Here are some representative Borogata type main keep. Aya Castle, in particular, exemplifies the look of a watchtower built atop another building.
|Aya Castle||Himeji Castle||Inuyama Castle||Kumamoto Castle|
Sotogata literally means multi-leveled tower type. This type of main keep is built when the foundation is nearly square. Each level is the same shape, simply a little smaller than the previous. You will not see any irimoya gables on a sotogata type main keep. The smaller gables you do see on sotogata main keeps are primarily decorative and are not a necessary part of the roofing construction of the castle.
Main keeps rarely stood alone and were often attached to smaller sections of yagura, small keeps, or large gates or walls to strengthen their defense. How they are connected to other structures determines the main keep style.
Compound - Fukugoshiki (複合式)The main keep is directly connected to a smaller tower or yagura.
|Inuyama Castle||Odawara Castle||Matsue Castle|
Adjoined - Renketsushiki (連結式)The main keep is connected to a smaller tower or yagura by a watari-yagura. I picked Matsumoto Castle below as one example of a renketsushiki main keep, but actually Matsumoto Castle exemplifies both Fukugoshiki and Renketsushiki. The Inui Kotenshu is connected by a watari yagura, making it renketsushiki, but the Tatsumi Tsuki-yagura on the opposite side is connected directly to the castle making it fukugoshiki.
|Nagoya Castle||Kumamoto Castle||Matsumoto Castle|
Complex - Renritsushiki (連立式)The main keep and multiple smaller tower/yagura are each connected by watari-yagura or tamon yagura to completely enclose a small courtyard or the entire honmaru bailey.
|Himeji Castle||Iyo Matsuyama Castle||Wakayama Castle|
Independent - Dokuritsushiki (独立式)The main keep stands independently of other yagura.
|Maruoka Castle||Uwajima Castle||Kochi Castle|