Order Units To Move From A Region To Another Region...

For easier explanation of my problem, I have attached an image.

As you can see on the picture, the units have been blocked by towers and cannot walk further, but blizzard made it the way that they destroy blocking things if they can. This is good, but when they have destroyed the towers, they don't keep going (just standing still) to the region they were ordered to.

My question is: How do you make them walk to that region after they have destroyed the towers?

Just pick the unit that attacked the tower, and when the tower dies, order the attacking unit to move to the nearest region. Or just detect which region it is in and tell it to go the next one.

How should I detect which region it is then? And go to nearest region don't work very well...