The zone for (16, 4) is bounded by walls
    0
    1
    4
    5
The zone for (12, 7) is bounded by walls
    1
    2
    4
    5
The zone for (18, 12) is bounded by walls
    1
    2
    6
    7
The zone for (16, 20) is bounded by walls
    3
    7
The zone for (16, -4) is bounded by walls
    0
    1
    2
    3
    4
    5
    6
    7