Pure mathematics is the world's best game. It is more absorbing than chess, more of a gamble than poker, and lasts longer than Monopoly. It's free. It can be played anywhere - Archimedes did it in a bathtub
You cant choose king region or a region without caballeros for example. Only region which can fulfill the action can be chosen
It will be a bit silly if you could chose king region as everybody will choose the king region and the card would do nothing.