Not sure if intended, but the wind generators on the hill stopped the starlight from digging through it.

Technically, nothing could stop the players from manually targeting the wind gens then targeting behind the mountain to dig through it, but it's odd that the cheapest building in the game combined with a bit of terraform/natural elevation can stop the most expensive weapon in the game.

I could be wrong, but I think it used to be that way (or perhaps still is in some way) and people would exploit that to push a wreck on an impossibly high spire, then revive the building. Something along those lines. I've never been quite clear on how that worked.