The Aim of the Game - for black
The King can be captured on the central throne square (left) or elsewhere(centre). Also, in the diagram on the right, the King has been captured on the square next to the throne by three black warriors.
Obviously the black player has to stop the white King from escaping to a corner of the board. But in order to win the game, black must also capture the white King.
The white king can only be captured by being surrounded on all four sides by black warriors, or by three warriors and the throne square.
Three warriors and the throne square? That's right, it's as if the throne square plays the part of the fourth black warrior for the purpose of capturing the king. The reason for this is because black warriors are not allowed to stand on the throne square, and so without this rule the king would always be perfectly safe on the square next to it.