Before it became part of Japan, the Ryukyu Kingdom stretched from Amami-Oshima (now part of Kagoshima prefecture) in the north to the Yaeyama islands in the south. The Ryukyu King resided in the castle of Shuri-jo, which made it the heart of the islands in every way, politically, economically and religiously. These days it's a UNESCO World Heritage Site, one of Naha's most visited attractions, and a good place to get a sense of Ryukyuan culture.
Shuri-jo is situated at the top of a small hill overlooking Naha, a few minutes' ride from the city on its fun monorail. It's the only intact gusuku, or Ryukyuan castle, though it's a reconstruction of the original structure, which was almost destroyed during the Battle of Okinawa. Its outer wall contains expansive grounds which are home to a handful of interesting sights; the Iri-no-azana observatory commands a fine view of Naha and the ocean, and there are a number of bright vermilion gates in the inner wall.
If you get to the main Houshinmon gate just before opening, you'll get to see the opening ceremony, performed by gatekeepers in traditional costume. Near this gate are a pair of buildings which served previously as offices, but now hold regular performances of Ryukyuan dance by performers resplendent in beautiful colored costumes (except for the ones all in black). These shows are held three times a day on Wednesday, Friday, Saturday, Sunday and national holidays.
It's inside the inner wall that you get to see what sets gusuku apart from castles on the Japanese mainland. The tiled surface of the forecourt is wildly striking with its red and white stripes, and the scarlet buildings on each side show how Japanese and Chinese influences fused to produce something distinct from either, their simple design enlivened by the bright coloration.
In the Hokuden (North Hall) you'll find exhibitions on Ryukyuan culture and a gift shop, but the main attraction here is the Seiden, or State Hall. The first and second floors served as the Ryukyuan kings' throne-room, depending on the occasion, and the lavish decorations and vaulted ceilings create a fitting air of authority, not least in the numerous paintings and carvings of dragons.
Within walking distance of Shuri-jo are two other World Heritage Sites; the Sonohyan-utaki stone gate was where the Ryukyuan king stopped to pray for a safe journey whenever he left the castle, while Tamaudun is the atmospheric mausoleum in which Ryukyuan royalty were entombed. There's also an information center and rest area, with another gift shop, and a restaurant serving tasty Okinawan food.