From the World Heritage inscription:
Five hundred years of Ryukyuan history (12th-17th century) are represented by this group of sites and monuments. The ruins of the castles, on imposing elevated sites, are evidence for the social structure over much of that period, while the sacred sites provide mute testimony to the rare survival of an ancient form of religion into the modern age. The wide- ranging economic and cultural contacts of the Ryukyu Islands over that period gave rise to a unique culture.
Okinawa is geographically and culturally different than the rest of Japan. I usually explain it as Japan’s Hawaii. Its language is distinct from Japanese and the islands weren’t even formally part of Japan until the 1870′s.
Much of Shuri-jo Castle and other landmarks in Okinawa were damaged during the battle of Okinawa in WWII, which took the lives of over 100,000 Japanese and 13,000 allied soldiers. The current castle is mostly rebuilt and reconstruction is still ongoing.
View my complete list of UNESCO World Heritage sites.