Alliance are a serie of different mods. On the Star Wars GC, you can play the civil war Empire vs Rebel era + Black Sun and soon the Republic and CIS.
At this battle you can see the walls with defense weapons. There will be a different design for the Rebels, Empire and Black Sun. This designs will be shared with other factions.
Before you think about it, it will be light weapons, they will have not the power from a normal turbolaser turret and they can not damage some special units as a AT-AT.
The foundation of the walls was still there after their destruction.
Are they rebuild-able for your faction?
It runs as another building. You will lose every the walls if all them are destroyed. If all them are destroyed, you can build them again from the galactic conquest.