When I think about it, it seems that controlling the space above a planet will be more important than I have thought. You see, if a planet loses it's space to the invaders, they've got their backs against the wall. No way to get reinforcements, stuck with the low number of units on the ground because of the pop cap. If you don't get the space back, you're doomed. No matter how many turrets you build, no matter how well you cover the reinforcement points, you're doomed, because the enemy can just poor in more and more units. This does make sense in a way, but the bad part is that you can't defeat your opponent by just defending your ground base. If the enemy is determined to get the planet, they will not stop attacking no matter what the casualties are. Seems like owning the space will be extremely vital.