The animal classes could use a little balancing. From what I can tell, there are only 4 magic animals, but 7 fire. Only 5 water, and 4 brute, but 8 shadow.
I wouldn't mind seeing a couple more magic creatures in there.
I agree about the lack of menu while in game. It'd be nice to be able to restart. Having said that, just right click and hit 'Play' to restart the game.
Okay, I just beat it on Apocalypse. Takes a while. It took me 4 tries before I figured out the strategy (different that what works on the other 3 difficulties). The only disappointing part is that once you get enough upgraded weapons, it's no longer very hard. BUT, it is hard to get to that point. Let me just say that bunkers are crucial in apocalypse...
 "Brute Wars"){kind=icon}
 "Castlewars"){kind=icon}
 "PlanetDefender"){kind=icon}
 "Snowman Attack"){kind=icon}