InvincibleMin and Max
Official maps use "InvincibleMin": [-1, 0, -1] to make the bottom layer of the map invincible.
Fields
InvincibleMin <position>
InvincibleMax <position>
Limitations
There is no way to make an invincible border around the map using InvincibleMin/InvincibleMax.
WORKAROUND
To make an invincible border, use a duplicate index with the invincible block attribute for every index in the invincible border.

