[geoschmo]

The biggest problem with defending a planet using weapons platforms is the fact that they have a built in multiplex of ten, that is, they can only target ten ships per combat turn. This is regardless of how many weapons platforms you have on the planet and how many waepons you have per platform, and is not improved by the multiplex components. While a fleet of 100 ships will be able to bring all it's weapons to bear on the planet on every combat turn. All that it can bring into range anyway. You would have to really beef up the weapons platforms to be able to survive a determined opponent.

You could build an uber-gun that would destroy any possible opponents ship in one shot, certainly reasonable in many sci-fi universes. But even then you'd only be able to take out ten ships per round. A large enough fleet could absorb that rate of attrition and still glass the planet in a couple combat turns. Unless you also jacked up the weapons platform shielding to make them significantly invulnerable to orbital fire. It would be difficult to balance.