I'm wondering, what null tile(s?) is walkable.
I know PCFredz just pinned an example of solving stuff for yourself, which I normally do, but I really don't have the time or energy to go through like a million tiles.
That's okay. It's the ones directly to the right of the creep tiles. It should be noted that you can't actually walk onto these tiles, you have to have the unit placed on the tiles with a trigger, but once they're on they can move around on them. Beware, they can actually walk over unwalkable terrain such as walls directly from the walkable null terrain.