Master Yen Sid’s Guidance
There isn’t much to do in the Mysterious Tower except collect Treasure Chests and a Sticker (though you’ll have to return later to get the Sticker). Open the three Treasure Chests in the “Mysterious Tower” area, then enter the castle. Another Treasure Chest is in the back corner of the “Tower Entrance”.
Climb the stairs to enter the “Sorcerer’s Chamber”, where Master Yen Sid has some guiding advice for Terra.
Master Yen Sid sends Terra on his way, which brings him back out to the World Map. A new world appears at the very top of the map, so go there to trigger a rather lengthy cutscene in which Terra finally locates Master Xehanort. The two of them talk for a while, and Master Xehanort has Terra convinced that the next logical world to visit is none other than the city of light, Radiant Garden.