From the the World Heritage inscription:
Himeji-jo is the finest surviving example of early 17th-century Japanese castle architecture, comprising 83 buildings with highly developed systems of defence and ingenious protection devices dating from the beginning of the Shogun period. It is a masterpiece of construction in wood, combining function with aesthetic appeal, both in its elegant appearance unified by the white plastered earthen walls and in the subtlety of the relationships between the building masses and the multiple roof layers.
Himeji Castle is a must stop if you are traveling on the Shinkansen (bullet train) between Hiroshima and Kobe. You can stop at Himeji, put your luggage in one of the lockers at the train station and easily walk down the main street to the castle. Himiji-jo is located on a hill in the center of town and is easily visible from the train station.
The castle was built to confuse potential invaders so the interior of the keep is a maze of paths. The castle was never actually attacked, so the fortifications went untested. Unlike many similar castles in Japan (Osaka and Hiroshima), Himeji Castle is the original structure and is not reconstructed.
View my complete list of UNESCO World Heritage sites.