I just cleared the fourth castle in Extreme. Awesome game, man. Keep up the good work :D
- Many towers are useless until they level up. The problem is, they can't level up if they don't kill enemies. Because of that you can't build many towers behind your current defense line, because they'll never get enough XP to be useful. That kills many possible strategies. Because of that I think you should be able to buy a level with skulls, maybe for 1000 or 1500.
- The mud golem is too expensive and too hard to level up. When you have enough skulls to buy it, you already used all your spaces with cheaper towers.
- The dragon has no clear use. It begins as a very, VERY expensive skeleton, and then it's a very crappy demon with some damage. I think it should be a tower with a large AoE effect, twice as large as the skeleton's, or maybe more.