First of all: really GREAT GAME and many THANKS for this post. I loved the first opus, the second one is even better.
I liked: music, sounds (the laugh when you plant or upgrade a tower could not be better!), TD/RPG mix, variety of levels, the story as you play a villain and make your way through different territories in a classic but highly pleasant fantasy world, the day/night missions, the medals system, the lovely graphics. This is a very accessible game in every way and yet manages to stay quite challenging in the final levels. Every details have been designed to be simple and effective. A great success I reckon.
I do not share complaints about last levels: on the very opposite! Amazing how Amazons, sacred stones or maps with little place for towers (and of course the day/night system) can impact the gaming experience. The 2 last regions are truly apart from the rest of the game, this may be disturbing at first, but finally I believe this variety brings a huge dimension to an already awesome game.
Also to say last levels are hard but can be made with upgrades. And I think this is definitely a good thing to be able to enjoy a couple of hard levels while fully built up instead of only squashing meaningless waves of ennemies without effort. I had a tough time on levels 18 (damn Amazons!) and 20 to achieve brilliant (by day only, by night surprisingly I managed to get this medal quicker). Of course, having an extra money and/or mana is sure helpful, but I would not say this is as critical as some comments pointed out.
LEVEL 20: I destroyed lower lighthouse, then upper lighthouse and upper boat (having enough mana left to cast at least a fear). For the lower boat, I just built a volcano tower near it at an early stage. After this, I destroyed the sacred stones on the left side of the screen and placed towers. With this strategy, you mainly have to watch for the divers. This may seem hard, so here is a tip: try to focus on the divers health levels and cast fear on the healthiest ones in advance, no need to always wait for them to be near the gems. Fear also works under water, so you can shoot multiple divers with 1 spell.
All in all a great game, very well thought and balanced, thanks again. 1 thing though, I would have loved a picture or even a little animation of the villain with all his treasures (like in the 1st game if I recall correctly) when every medals collected!!