Castle is a property of your kingdom based on things you have that raises it. Castle goes up and down depending upon what things you have on your map that boost it. It cannot be exchanged. It is used to unlock map expansions which have a minimum castle requirement to expand into.
Ways to get castle include
- Completing Royal Buildings (these cannot be sold or stored, max of 2 each)
- Placing Royal Decor (these can be sold or stored)
- Placing Walls (these can be sold or stored)
The amount of castle a player has also determines where they appear on the list of kingdoms at the bottom of the screen. It is arranged left to right from least to greatest amount of castle. XP and Level do not determine this order.