This is by far the best tower defense game I've played; a genre which is otherwise not exactly my cup of tea. I really like the many different skills and level layouts and I find the progression rate reasonably balanced, which is crucial to an enjoyable playing experience (i.e. grinding necessary kept to a minimum). I do think that the theme of each land could be more distinct from each other, maybe in terms more enemies being particular to one land and different theme music, which would also bestow more variety on the game, dunno. Other than that I am really enjoying the game. Highly addictive!
Kudos for the good work, guys - keep it up!