Learning ... wall.
I was able to get through missions up to this with little difficulty. There was a nice learning curve as I adjusted my tactics a bit to the point where I was able to beat many levels without hardly touching orbital abilities and kept them in reserve to reinforce my towers. Then suddenly Mission XIII happened and the learning curve became bashing my head against the wall of a sheer cliff. I've replayed it multiple times and only made some progress toward acing it, like all previous maps, until I immediately rushed so I could afford to upgrade my command center.
I've tried different tower configurations from spamming weaker towers as fast as I could afford them, to building more powerful towers where they can hit enemies on more than one path, to everything in between that I could think of. I think this level needs to be rebalanced because the 'normal' difficulty does NOT fit with the difficulty of previous maps on normal. I know some would counter with the idea that I just need to find a way to more cleverly use orbital abilities ... but elitist mentalities don't line up with the mentality of most people that would be playing the game. Each level, in my opinion, needs to be winnable by a mixture of strategies to fit the play styles of gamers at large that may not stick to the game so obsessively.
... I cringe at the thought of having to beat the level on easy to ace it ... Through the many tower defense games that I've played, I have only had to do this on a very rare occasion when I still had a large amount of tower buffs to unlock, like in Kingdom Rush, and could then go back to ace the level with well-buffed towers. I don't think that would apply to Siege of Centauri so much as it doesn't have the same kind of 'buff every aspect of every tower' approach.