Castles During the Sengoku Period:
During the Sengoku Period (Warring States Period), literally hundreds of castles were constructed throughout Japan. The first castles were built high atop inaccessible mountains to serve as both a watchtower over the land and a safe retreat for the daimyo during a war. These castles are comparatively smaller and do not maintain as extensive moats, turrets or other supporting structures found in other castle types. These are called yamajiro(mountain castle).
Iwakun and Gifu castles are prime examples of yamajiro among the castles I've visited. In both cases they had ski lift type ropeways to carry visitors to the top of the mountain. When I visited these castles I was impressed at how well they built such a fortress in a nearly inaccessible location.
As the Sengoku Period got underway it became necessary to not only conduct battles from the castle but also to administer the government, manage growing armies and attend to the ever changing web of alliances. The mountaintop castle was therefore faded out in favor of more practical designs. The hirayamajiro(flatland-mountain castle) was built on a low hill with a good view of the surrounding area. The main keep of these castles was built larger and taller than the mountain top castles providing a good vantage point to watch over the land. Oda Nobunaga built the first such castle, Azuchi Castle, in 1579. Azuchi Castle was destroyed after Nobunaga's death in 1582. With the change from mountaintop castles the main keep of flatland castles also became more beautiful and grand.
Sengoku "Warring States" Period (1467-1615)
The Sengoku Period is usually defined as the years from 1467-1573, but I would like to loosely define it as the entire time period from the Onin Wars until Tokugawa Ieyasu consolidated his power in 1615.
The Sengoku Period is one of the most interesting periods in Japanese history and gives rise to some of the most famous historical figures. It is the golden age of the samurai. Following the Onin Wars (1467-1477), Ashikaga rule weakened and Japan was subsequently thrown into a century of anarchy.
Political forces and alliances changed regularly and everyone continually vied for more power and larger territories. Into this chaos marched Oda Nobunaga. Utilizing brilliant military and political tactics Nobunaga began the unification of Japan. He was, unfortunately, forced to commit suicide by one of his own lieutenants, Akechi Mitsuhide, in 1582. Nobunaga's top aid Toyotomi Hideyoshi picked up where Nobunaga left off and completed the unification of Japan.
Hideyoshi lived a colorful life indulging in the arts and also waging two disastrous invasions of Korea. Before his death in 1598, Hideyoshi established a government ruled by a council of lords in place of his young heir Hideyori.
A complicated power struggling involving the lords of Hideyori's council ended in the Battle of Sekigahara in 1600. Tokugawa Ieyasu, a member of the aforementioned council, emerged as the victor and moved the military capital to his castle town of Edo (Tokyo).
In 1603 Tokugawa was named Shogun by the emperor which gave him undeniable supreme military command over the country. In 1615 Tokugawa stormed Hideyori's fortress of Osaka Castle and eliminated the only remaining threat to his power. Tokugawa died a year later in 1616 and the Sengoku Period came to a close.