Ghorin's castle.
During the Galactic Civil War, Overlord Ghorin lived there and ruled the Greater Plooriod Cluster from his luxurious mansion, which was built upon one of the islands.
In 0 ABY, Overlord Ghorin was executed by Darth Vader in his mansion for allegedly providing poisoned grain for the Empire. He had in fact been framed by the Rebels as retaliation for supplying them with poisoned grain.
The Complete Star Wars Encyclopedia, published in 2008, has an entry for Overlord Ghorin which refers to his castle on Plooriod IV. This contradicts the Imperial Pursuit cut scene, which shows the castle on Plooriod III. Anyway, the Plooriod III's and Plooriod IV's entries are correct.
- The Complete Star Wars Encyclopedia