I am a fan of tower defense maps ever since WC3, and I enjoyed the hero defense style as well. This game gives a pretty solid indie hybrid of the two with one flaw that is sometimes hard to ignore, but otherwise is pleasurable for a number of weeks (in my case). Leveling is interesting, giving you the ability to increase stats for either your hero or your towers and even allowing you to respec with a reasonable cost. Some of the balancing techniques take some getting used to (like starting out on level 5), but they give you enough to work with in all situations.
I did mention a flaw; the camera sucks. There is no way around it, the camera just plain sucks. After having played Renegade OPs not too long ago, I no longer feel like I should settle for my camera angles. Their dynamic view is one of the coolest things I have seen in awhile.
The loot is fun and items are upgradable. You can get pets, which can either attack or give passive bonuses and can also be upgraded. I enjoyed playing this game co-op local with a friend, and find the benefit of having more than one hero active on the map at a time for repairs and upgrades during the combat outweighs the split screen.
The towers are interesting because they vary wildly between the four classes; standard projectile based towers to the more advanced (and possibly less effective) aura based towers which constantly need to be repaired, but add a new dynamic to gameplay and breaks up the tedium of certain waves.
For the price, around $15 USD it was an extremely satisfying game. Especially because my friend paid for it. Goodnight, I'll be here all the week!