Guys, he's clearly talking about those two unfinished buildings to left on the screenshot, there's clearly a path to the tower that has been blocked by other buildings, what's so difficult to understand lol?
That said while in map editor you can press the keyboard button "p" to see the pathing colors on the map to make sure that the terrain pathing is correct.
Anyway, my attempt to help:
Only way I can think of is to somehow figure out the pathing (not including buildings & units) and then figure out which buildings or units has to die that would clear a path to the tower within the shortest time, i.e. for simplicity's sake: least number of blocking buildings required to kill (so not counting Effective Hit Points, distance to move, etc), and then order the creeps to attack that building.
How you would do that I've got no real idea.
What I'm guessing is that there's some way to change the unit behaviour, since that should be way easier, try stuff like removing the computer AI from the creeps, trigger action - ignore unit guard position, object editor unit data - unit can flee = false, and whatever else you can come up with.