I think doubling is good because the curve grows fast so you shouldn't have to go to every planet and click too many times to get the defense that you want.
Again, I think the most intuitive method of dividing resources spent on defense is via the fleet interface. If you want to defend a system, move some ships in. If you want to double the defense of that system, move twice as many ships in.
I'd still like to see defensive structures of some kind. They would basically be ships that cannot move. If we only use ships for defense, that cuts down on the fun.
If there is a defensive structure, I'd want it to be something like Smac's/Civ's citywalls. If you need more defense on a planet, perhaps because it's a fortress world meant to defend a chokepoint or the location of an important shipyard, then the player would use another build slot to construct a second defensive structure of the same type.
Rather than upgrading the structure itself, you'd research defensive technologies to improve these "citywalls."
Wouldn't be less micro to not have to build the defense structure a second time. So if we upgraded the structure, at least then we could have it done automatically based on some kind of defense emphasis we set for the planet. Set it to high, it upgrades faster and builds more defense structures itself.
Instead of directly attacking enemy ships, these defensive structures would increase conquest time, cut down on piracy and other bad random events, and mostly importantly aid friendly ships in the system, giving them a bonus in combat. Because the structure would be essentially useless without friendly ships, it enforces the notion that starships are the one true gauge of military might.
Maybe it's not terribly realistic, but I think it would make for cleaner gameplay.
Yeah, but that sounbds a little boring to me. I mean I would like to see the possiblilty of going into a system with no defending ships and only the planets defense type buildings. If all a laser battery did was give my ships in the system a bonus to attack, well that is pretty lame. I would be much happier if it did what I expected and fired lasers of its own.
Misc. structures that help with defense without directly effecting combat, such sensors that spy into neighboring systems or gravity wells that mess with the starlanes, seem far more interesting to me than messing around with kill-o-zap buildings--especially since we've already got ships to do the kill-o-zapping.
A few rare defensive Wonders might be cool too, but they should have an area of effect. (like the Great Wall of China project in Civ)
You have to have a variety though. If we threw out the defense structures that 'just shot lasers' because they are too normal, then that is not good at all. I would expect the kill-o-zap buildings to be just as useful as ships. And I don't see a reason to phase them out because 'ships are better'.