Awesome
This is a great TD game with lots of new features, good graphics, interface, and game balance. Probably the best TD I've seen in a while.
I've gotten "Brilliant" on all levels. For ninjas, you need towers at the entrance to trigger their smoke bomb, then towers at the gem so that they are killed when they become visible again. Meteors can be used as backup. For ninja champions in lvl 14 and 15, the only way to beat them without having them touch your gems is to place only chilling crypt towers at the entrance, so they can be frozen/feared when they enter. Then, you can take care of them. Chilling towers can miss the fear, so there is some luck involved.
As many have said, the crypt tower is the most powerful and versatile. The only thing it's lacking is quick damage (at the entrance, and to kill ninjas at the back).
For support damage, the den and temple are reasonably interchangeable early on. Later, the den (especially the orcish den) is much more powerful than the temple because (1) dens benefit from increased fire rate, so damage output is higher and (2) can kill ninjas in one hit. So these should be considerations for investing points into skills. If you just want to play safe, the temple's burning gem and gem return skills are useful in the short run, but the den's forestry skills allow you to play more aggressively.
Of course, the best upgrades in general are the XP/Cost upgrades