Skills should have upgrades as well. There should be more information on the stengths and weaknesses of various units-both friendly turrets and enemy. Perhaps a unit enyclopedia of sorts? For instance it takes playing through the game a few times to know even halfway what all the turrets do and some of them are not really worth unlocking at all depending on your strategy. Also it is annoying that I am very deep in the game and keep getting asked to unlock the Aegis Barrier. I don't want the Aegis Barrier. I have to click on it in very battle just to turn it down so the pop-up window doesnt distract me. I am only giving feedback because I love this game!
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