Complex and challenging
-I thought gem-matching was critical until I discovered that it was cheaper to just spawn them whole. Then I couldn't progress, until I noticed how powerful gem-bombs could be when you put some skill points into it. Then I went back and got glowing frames in the first few stages, only to discover that that shelling the enemy into oblivion is too costly in the later stages. The first epic stage Boss kicked my ass over and over again until I learned the foresight to max my Mana Pool early and the patience not to build anything during the last few waves. And when you finally start maxing those top-teir skills, everything else fades into the background as you focus on building a ludicrous arsenal of grade 7 gems. From that point on, it's pure Tower Defense, but the route there is delightfully diverse.
-My one complaint was that the most interesting play mechanic (matching gems) goes under-used, since you can buy gems cheaper than you can make them. Don't think of this game as a hybrid between gem-matching Puzzle Game and Tower Defense. Instead, think of the gem combos as a way of optimizing your inventory. It's a means of buffering and condensing your offensive power into a few key towers. Basically what this game does is give you plenty of options, and then force you to experiment with them, since the optimal technique changes as you level up. I happily invite sequals and rip-offs, because I think there's so much diversity here, further experimentation couldn't damage the overall quality.