The king can never move to a square that an enemy piece attacks, and the two kings can never stand next to each other.
What you learn here
The king ♔ moves one square in any direction. It is the most important piece: you may never make a move that leaves your king attacked. That is why it cannot step onto squares the enemy controls.
Chapter 1 — Meet the pieces: Every piece moves in its own way. Tap the squares it can reach, then guide it to the star.
