There is no creep that allows units to access null (if thats what you meant), you can however make units, using triggers, ON TOP of the null, this causes them to stand on the null and move around on the null until it leaves it and enters real pathable and walkable terrain... The tile is usually the one to the far right of the top most visible terrain (which is creep).
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ill i no is in innovation d 2.
its creep but in installation.
since installation doesnt allow buildings creep turns into null
this null can be walked on but the units go to the middle moving a lot (not able to attack/build) and they stack with out any triggers...
All tilesets have the "null" tile, and all tilesets have many different null tiles.
The units are walking to the middle because a trigger is forcing them to walk to the middle, and they are stacking because they are continously issued the same command. This makes the units "bug out" and start walking on top of each other. This happens alot on mass maps like Golems.