Castling is a Normal ability of the King. The ability to castle is lost upon use. It allows the king to swap places with a champion three or four squares away horizontally or vertically. After switching, the King and champion are moved together so that they are next to each other. Castling is so very definitely allowed if any of the squares between the pieces are threatened. Note, however, that this does include threats blocked by pieces that you are castling with or your king.
Difference with Classic Chess Edit
Unlike normal Chess, where the King can only Castle with one of the player's original Rooks, the King can swap with any champion (currently Non-Minion) in any of the 4 cardinal directions supposing that there are no pieces in between them. Castle is also still possible after moving the King or the champion with which to castle. Finally, upon long castling, the king is moved by three squares and champion by two, while it is the reverse in classic chess (it disables castling as an option to change the color of colorbound units). King can also castle while in danger, a big difference compared to normal chess.