You only need two copies to Upgrade a Prince or Princess.
Sky Castle is a Level 4 treasure in Storybook Brawl.
Prince / Princess builds are very popular at the moment, personally I find them boring. However if you are playing one then this treasure is a must-buy, especially following the 0.68.9 patch.
Game Update History
0.68.9, March 2022: Text changed from “Your Princes and Princesses have +4/+4.” to “You only need two copies to Upgrade a Prince or Princess.”. New artwork.