The skill tree gives an incredible amount of freedom, and lets makes your choices be important because you cannot reset. Each tower has two different paths that can be upgraded. It was fun to play and get all the badges. Most difficult level is level 14. That ninja boss is downright lethal. ~2250 health essentially makes you have to damage it a little then spam meteor to succeed.
Strategy guide? I found that going down Undead helped a lot. It promotes incredible mana efficiency that will help later, and the +25% fearing and 25% freezing helps out a lot against bosses, ninjas, and assassins. I next went down to the demons tree to get meteor mastery and meteor study. With the undead talents + these you can easily own protect your gems with meteors. After that I went into orcs to get the Cut Down Study/Mastery. With that, adoration, meteors, high mana you can essentially make as much money as you need and stop any foe.
i have always enjoyed this game. its all in the prediction. and people who are complaining about missing, it happens to everyone. even your opponents. you arent special