Love these games!
I've always been a huge fan of defense games (especially ones like this and age of war) so I was delighted to spend however many hours of my day beating this. Like you said, very addictive. I love the range of upgrades/skills you can get, and the logical fact that you can't buy skill with gold. I liked how you used exp instead. When it comes to improvements, I agree with Gerontius (previous reviewer) about how you can get better than a perfect score and possibley getting an auto-fire option.
I liked how the game wasn't too easy, but not too hard. If I could rate the difficulty, I would give it a 4 (1 being easiest, 10 being hardest) because after you get all of the crossbow upgrades, you can hold the enemy army at bay easily (as previously mentioned). But that wasn't a problem for me as I loved seeing the enemies fly into the air and exploding like fireworks when hit by arrows :D
I liked how the final battle had a boss at the end. I think that bosses at the end of certain levels throughout the game tends to ruin it. The boss forced me to replay other levels until I had enough exp to get every upgrade and skill to defend against it 'til the very end.
You put in a good range of of creatures to summon (but shouldn't the golems be on the enemy side and the centaurs be on the player's side? Just a thought), and just the right amount of skills to use. I felt like I had a good range of arrows to shoot and traps/turrets to lay out but didn't feel overwhelmed with the variety.
Probably my only complaint would be that the game verged on tediousness at the end although I can't really think of a solution for that.
It's always great to find a good game on newgrounds and I hope you make more!