I was pretty against removing PDC's, but the more I think about it, the more I like the idea.
Consider this: No two fortifications in real life are identical. So why, in VB6 Aurora, do we build PDC's following standardized designs? Why is a PDC on Earth, which presumably could be 10km under Mount Everest, equally armored as that same PDC on some Oort Cloud dustball that's barely 1km across?
The only solution to this I can see, is to abstract away the design of the fortification and make terrain and geology factor into its stats.
But this does raise a question. Will StO units be able to fire on targets on other bodies? I'm thinking of a laser battery on Mars frying targets on Phobos, or visa versa.