Crowns are the premium currency of CastleVille Legends, like its predecessor before it, CastleVille.
They can be earned by completing Quests, watching visions in Magnus' crystal ball, or purchasing with real-world currency.
Also, from a crown tree , of course you need to water it timely but you need to have 480 crowns to establish it. (no-kidding)
Also, from the jeweler for 250000 coins per crown. (not too small i guess ;P)
Crowns are used to supply missing resources, speed up crafting or harvesting, purchase additional Crafting Buildings, speed up Sea Trading, or even purchase pieces of stained glass windows in the Hall of Heroes.