Castles in Japan, like castles anywhere, were built for two main purposes. The first was as a defensive platform. Daimyo (feudal lords) all over the country built these fortresses where they could retreat during an attack. Both the castle itself and the grounds immediately surrounding it are fortified with a myriad of defenses. The main keep of the castle also held stores of food and weapons in case of prolonged battles. The second purpose of a castle was to display the daimyo's wealth and power. Naturally, the bigger the castle, the stronger and wealthier the daimyo.
The history of castles in Japan is intimately intertwined with the history of Japan itself. Castles have their roots in simple fortifications that were built around the homes of warlords. For nearly two centuries preceding the rise of Nobunaga, Japan was divided into a great many small provinces each ruled by a different warlord. As warlords fought with each other and gained more land, the need for bigger and stronger fortifications arose. Once Nobunaga started to unify and rule over larger sections of the country we start to see the emergence of the magnificent castles we think of today. Once the country was unified in peace under the Tokugawa the usefulness of castles declined and they became relics of a time passed.
The National Castle Administration Council ( zenkoku joukaku kanrisha kyoogikai) generally refers to castles by the city in which they are located. Thus the castle in the city of Himeji is called Himeji Castle. In the list of castles in the Castle Guide I chose to follow this convention and named each castle according to the city where it is located. I think this will also help you understand where each castle is geographically located. In some cases, however, the castles were not historically referred to by this name. The common name is given in the "alternate name" field of the castle description.
In Japanese, the suffix -jo on the name of a castle literally means "castle." In the list of castles, I named each one in English (ex. Himeji Castle), but in the text I chose to use the Japanese name (ex. Himeji-jo). Just as you would not call Ginkakuji, "Ginkaku Temple" simply because the suffix -ji means temple, likewise, I think it is natural and appropriate to name castles in the Japanese way. In fact, the official Library of Congress Subject Headings also list Japanese castles with the suffix -jo as opposed to writing out castle in English.
Names of people are also written in the Japanese way with surname first and given name second.