So I'm usually all for not coddling the player through things that should be obvious, but it's not exactly obvious what the upgrades do. At the moment I'm at a point where the opponent starts with twice as many units as I could *possibly* have and there's no clear way to leverage my abilities to make up for this. I like the concept, but the implementation is a few steps short of making this enjoyable for me.
COT 5 is hard, but not by any means THAT difficult. Just send in units with stun (mermaids, centaur archers, fire succubus, cannons) and units with the highest combined dps (Goblins, oddly enough, have a crazy attack power with 35 of them) and you're fine.
I think the game is well-thought out for the most part, though I don't like the new upgrade system. Could just be me, but building up every time is really getting on my nerves.
Great game, not too challenging though. Just grab the extra cash+luck early, and make it through to upgrade speed+5 and power+5. That's about the best strat I've found.