I would like to leave a more full review, but I'd like to add in on ArcaneHowitzer's bug; I also am unable to get stage 4's achievement. I assume that it didn't mean to not let any enemies through and instead meant to not let any enemies kill the engineers. I have done it every which way; no enemies through nor enemies let to engineers, still haven't been able to get it.
The game is pretty great, the early game however is completely dominated by archers. You can beat anything prior to level 8 with one or two Nott's and not have to worry about a single enemy getting through. The game play is pretty simplistic; I haven't looked into it any, but I would not be surprised to find this game in the app store. The game play is pretty polished, the only thing I'd like to see would be the tutorial-esque menus explaining the win conditions not show up /every/ time you replay a map. Once for each mode is more than plenty.
Art: 7/10. It has a cohesive style to it, and though it's a bit simple, it does of course have a charm to.
Audio: 5/10 Unforunately I had to turn the audio off fairly soon into the experience. It's too loud and there didn't seem to be a way to reduce it other than to flat out mute it all.
Gameplay: 7/10 The gameplay isn't amazing, but it isn't anything I feel the need to show my friends either. It's a simple top down arcade-y tower defense. You can't muck it up too bad, but there isn't much room for innovation in the genre.
Overall: 6.5/10
I hope to see some more uploads in the future! Also I hope that the number ratings arn't confused for what most people rate things as, where anything under a 7 sucks. Anything under a 4 or a 5 you'd legally have to pay me to play, anything over 6 or a 7 is in my bookmarks.