The Sanctuary is the Hub World in the 3DS version of Skylanders: Spyro's Adventure. It is the only place that Hektore can't reach the protagonists. The Skylanders can access any of the worlds from here, including Adventure Pack levels. You can also change your team of Skylanders by speaking with Wendel.
There are six different worlds accessible from the Sanctuary: Brighthold Battlements, Rivenrock Caverns, Faylair Jungle, Whitefall Summits, and Galecrack Ruins. Each of these worlds contain five different levels. Five Radiance Crystals can be obtained in each level.
There are also four other accessible worlds unlocked through Adventure Packs: Pirate Seas, Empire of Ice, Darklight Crypt, and Dragon's Peak. Each of these worlds only contain two levels; however, you can still get five Radiance Crystals per level.