i also played this one (hello from drill game!) and agree with a lot of nix's thoughts here.
only thing i would add is that this game is also SUPER close to being 100% mobile friendly. the only exception is the skill tree. there isn't a way to view what a skill does from mobile without buying it on click. i'm not holding this against your design or anything, but from what i can tell a little confirmation dialog box when selecting a skill would make your game completely compatible with Chrome for Android.