The Ruins, also known as Eon's Citadel is the main hub world in the console versions of Skylanders: Spyro's Adventure. It is what remained of the citadel that surrounded the Core of Light after it was destroyed by Kaos, and it is the main headquarters for the Skylanders. You come here after each level to upgrade the skills of your Skylanders (when unlocked), and complete Heroic Challenges (also needs to be unlocked).
As each of the components of the Core of Light is restored, the Ruins will gradually change in appearance, becoming more lush and vibrant. As of Giants, the Citadel was partially rebuilt, though a few buildings were still missing.