Heaven Corridor Guide (Obtaining Cecilia's Dual Cast)
After you obtain the Protowing you'll be able to access Heaven Corridor, a dungeon that's west of Adelhyde and north of Curan Abbey. It's blocked in by mountains and by ocean and difficult to spot unless you're flying in the area and know what you're looking for.
Inside of this dungeon you'll be able to obtain Cecilia's final force ability, Dual Cast. This allows you to cast two magical abilities during the same turn - which is extremely useful for some of the more difficult fights in the game. For the most part, this dungeon is pretty easy to figure your way through.
To gain access you'll need to use one of your Duplicators on the front door and then use your Tear Drop a little ways into the dungeon which'll teleport you into the skies where you'll find the 'meat' of the dungeon. The second Duplicator door you come across will have two Crest Graphs in it, to the east of this door you'll find a room with some vases which you'll need to hit a switch in one of the vases to enable further progress in the dungeon.
It's fairly linear so I won't take too much time breaking this whole dungeon down for you, most of your time spent in this dungeon will probably be getting teleported back to the entrance and having to run through the whole dungeon again. There are some really annoying ground teleporters which portal you back to the entrance when activated. You can't see them until it's too late then you have to memorize the location so you dodge it next time.
Towards the end of the dungeon you'll come across three puzzles in a row. Each of them involve a room with two conveyor belts, a small outcropping or gap in the blocks and a floating crystal that mirrors your every move. Your goal is to use the conveyor belts as well as unique design of the blocks to trap the crystal and touch it.
The first room is simple, use the upper right hand corner where there's one block sticking out to trap the crystal. Second room is shown in the screen shot above, as you see in the picture you'll want to use the opening in the middle to trap the crystal. I used Rudy's Skates and ran the opposite direction on the conveyor belts to trap the crystal easier in both room 2 and 3.
After you complete all three of the Catch-A-Crystal Puzzles you'll find yourself in an outside area again with multiple stair cases. At the base of the final staircase, between the two trees - shown in my screen shot above - you'll want to use the Tear Drop to call forth Dhee who will announce you to be his successor. A brief scene will commence before Dhee gives Cecilia her final force ability, Dual Cast. Once you receive this you can leave the dungeon as there's nothing more to do here.