Very good game for experienced Tower gamers
This is certainly a very hard game, though I got lvl 60 when I was in Hell Level. Although I have to mention that by lvl 57 the basic zombies were too powerful it was impossible to kill from there on. Animations were not the best, but not the worse. What's important is the gameplay and I thoroughly enjoyed it that I restarted every time I screwed up the steps to build a good defense. It would be good to have a tracker to at least allow me to see what my personal best is. Hope that'll be a improvement later on.
For the guys who cry that it's hard even in Easy: Build only one or two turrets at the start and upgrade the first front tower, as you gain more money, build around 2-3 towers before upgrading. After about 3/4 towers, I rush upgrade one tower (especially the chaingun tower, that is the best)