I observed tower stacking today. Player(Vier in replay) spawned at 3rd position and was able to built huge amount of Perpetual towers. He built Light towers, then upgraded them to level 2 and then to Perpetual. In the end he then built 4 light towers at the same spots and upgraded them to level 3. I have these 4 spots selected in this screenshot.
I have a replay uploaded here, but my SC2 crashes at 10th wave. :|
I can't give you anymore details because I can't watch the replay myself and I haven't screenshotted ingame because I thought I would select all them in replay for better showing.
