Japan's years of feudal lords and civil wars resulted in the construction of large numbers of castles, designed to withstand attack by cannons (gunpowder, remember, was invented in nearby China). Surrounded by moats and huge stone walls, the castles were intended to protect the local shogun's realm by providing samurai warriors with military supplies and a place where they could hold out during a siege.
However, during the 17th century the Tokugawa shogunate, cementing their centralized hold on power, ordered that each realm could have only one castle, so many were destroyed. Others fell into neglect or burned down. In 1873 the Meiji government decreed that another 144 be destroyed, leaving only 39. Most of these, including Edo Palace in the Tokyo Imperial Palace grounds, were destroyed during World War 2, reducing the number to 12.
In the 1960s, many castles were reconstructed out of concrete, however the original castle at Himeji remains to this day, and is universally acknowledged to be the best of the authentic structures. The castle, built in 1580, is called "The White Egret", because of its colour. Not only is it worth seeing, it's also easy to see, since it's only a fifteen minute walk from the Himeji train station, which serves this city about half-way between Kyoto and Hiroshima. It makes a good day trip, or a convenient stop between the two cities.
The castle exterior is magnificent, built on a small hill just above the surrounding plain.