I might be too late, but I finally got to play a few levels, so thanks Toma again
My experience with TD games is not much besides playing games like OMD1-2, Revenge of the Titans and, the most simple one, Go Home Dinosaurs
So here are my quick impressions. Like many said unlike most TD games, in this one you have to be very active when managing your resources between your defenses. In others once you get your killbox going you are almost set, not here. You also need to collect gems and some squares that work are currency but there is a cooldown time before you can set the same sentinel again (if I'm not wrong). Of course as you progress you can upgrade not only your units and get new ones (which affect a certain type of virus) but you can set units which increase how much power you have to distribute between the others.
I really like the visual design and the music, especially the latter, which made me want to keep going in a similar fashion as Hotline Miami's music did. I also liked how the music volume increases when you assign more power to the towers, nice little touch.
At first it can be difficult since the game doesn't explain itself too well but once you get the hang of it, it is very enjoyable. I played only 1 bonus level in my short time but it kicked my ass, I need to invest more time in it.
Hopefully that's useful for some and I'm clear enough since english is not my first language
