Ah, yes. What you need is an UNBUILDABLE pathing map. So the creeps can walk over, yet it will give you a 2x2 pattern where you want to place it.
You can
1. Find a doodad with a 2x2 unbuildable, but walkable pathing texture, and use that.
2. Blizzard has a BUNCH of premade pathing maps, which are used for the doodads. Change the pathing texture field, and in the box below enter:
PathTextures\2x2Unbuildable.tga
Pathing maps are JUST tga images. 2x2 squares is TWO pixels by two pixels. Its so small in file size. Each pixel on the TGA represents one square in the game. (a tower is 2x2).
If you want no real pathing, the common map used is MUSHROOMS.
The COLOR of the pixels determines how it behaves, which can be any of these:
-Air
-Ground
-Both
-Walkable
-Unbuildable
-Line of site
Knowing this, you can create your own pathing maps when your desperate.
I made a walkable map for the platforms in my mini game.
I adjusted the pathing map of the stone bridge short, so it will sit central in my map instead off offset one square.
I had MASSIVE ammount of blockers in my TD at the sides of the lanes. I created my own giant strip blocker, and reduced it by hundreds.