For your next game maybe you could add different difficulty options.. I had to play level 13 four times before I won, and that's why I LOVED it. It will be a shame for me if you reduce the difficulty before I complete the game.
In response to the question "Do we really need the feature of removing individual upgrades instead of resetting all of them". That depends on how you define "need" :p but since in this game you start every level with different unit types, it's very useful to constantly change your upgrade choices, and being able to change them individually instead of reset all would be useful.
By the wat I found level 12 quite easy.. I don't know if you have already updated, but it was just a case of spreading out my units a little bit to stop the hounds running past.
I think there is a mistake. The new enemy on level 12 is described as a "Grey Hound.. bigger and tougher than Green Hounds.." but those are clearly Green Hounds, not Grey Hounds.
Generally nice update. I don't know why you nerfed the Spectre though. It's the coolest unit in the game and now it's redundant. Multiplayer will never be ballanced because you need a Blizzard style test team working on it to ballance a game with so many units.
I'm starting with 100 less energy units today.. It must be an update but it feels a lot more like a bug when you are on level 15.
Tell me the enemy has also been nerfed..!