Knight#valid_move? could potentially be simplified

[olleicua]

I think the approach you have used here makes more sense for the knight than it does for the king but I think there might still be a simpler way. It's up to you how you want to do this but consider the following:

class Knight
  def valid_move?(x, y)
    dx = (x_position - x).abs
    dy = (y_position - y).abs

    super(x, y) && [dx, dy].sort == [1, 2]
  end
end

Also you don't need to call within_chessboard? as that is covered by super