What are your opinions about score unlocking upgrades?
Hi I played the first 2 missions of the game. It seems your total score over all missions determines when you can unlock higher level towers (you start with Level 1, then you have to earn so much score to be able to upgrade them).
That means there is a reason to beat the missions first on not maximum difficulty, then when you are prepared replay the mission at maximum difficulty later.
My opinion is that it is much better than having to spend metal instead to unlock upgrades, because balancing getting across-mission upgrades against getting more towers in this specific mission would be frustrating. I also believe it is also better than spending research-only resource (like crystal in Sins of a Solar Empire) that you earn by mining the map, because trying to squeeze out maximum mining efficiency is not as fun as ensuring you destroy all enemies and complete all objectives.
The only mild disappointment I have is that replaying missions at higher difficulties pads out the game. I would prefer spending that time playing new missions, but I understand that this is a low-budget game.