You'll be facing a fair number of attacks where a "rolling defense" approach can be helpful, meaning using border defense worlds not for the purpose of totally stopping an attack but rather taking enough wind out of its sails that your further in planets can stop them, etc. I've found that grav turrets are wonderful for this. Riot Control Starships and Spider Turrets are also great at whittling away at enemy numbers (in the lead force, the actual dangerous part) via engine damage.
Also, one of my main defensive insurance tactics is layering TONS of forcefields on top of my home command station, and that basically means ff II and ff III unlocks are a given. Also, those unlock higher level shield modules for the fallen spire stuff.
On that note, laser turrets and heavy beam cannon turrets also unlock some higher level FS techs (so does spider turret and the siege starship techs, but those are more optional depending on how you want to outfit your spire ships and cities).
Defensively, I highly suggest sticking your spire cities on top of wormholes that lead deeper into your territory so that their shield generators will prevent entry into (as opposed to exit from, which doesn't work because the AI ships just pop out outside the shield).
