If you choose to go the free route, you'll be able to complete all six levels of upgrades by collecting parts from friends. Here's a rundown of each stage and the building materials necessary to complete it, thanks to Zynga.
Stage 1: 6 Hover Charms, 6 Cloud Bricks, 9 Enchanted Bamboo
Stage 2: 8 Hover Charms, 8 Cloud Bricks, 12 Enchanted Bamboo
Stage 3: 10 Hover Charms, 10 Cloud Bricks, 15 Enchanted Bamboo
Stage 4: 12 Hover Charms, 12 Cloud Bricks, 18 Enchanted Bamboo
Stage 5: 14 Hover Charms, 14 Cloud Bricks, 21 Enchanted Bamboo
Stage 6: 16 Hover Charms, 16 Cloud Bricks, 24 Enchanted Bamboo
If you do decide to build more than one of these Floating Castles across multiple farms, you'll need to keep in mind that you won't have your building progress carried over from one to the other, and must build each Castle individually, collecting building materials for each.
As you build the Floating Castle, it begins to function as an item collection spot. You'll be able to collect from it to earn gems for the Gem Tower, along with Special Delivery Boxes that will (hopefully) contain the items you need to complete other features in the game.
This Floating Castle will only be available to place on your farms for a short time, but will be around to harvest and build long after the base has been removed from the store. That being the case, you might want to place a base on each of your farms, just in case you'll even want a Floating Castle there, since you eventually won't have the option to place them anymore. Good luck building this and any other item you're still trying to complete on your farms!
What do you think of this Floating Castle? Have you already started building one on your Jade Falls farm, or are you building it elsewhere? Sound off in the comments!