This is one of those games that I realised half-way through I was only playing it to get to the end; I wasn't actually enjoying it all. The reason I think is that, unlike a true tower defense game, you don't get the opportunity to carefully plan your defenses and a clever route and then sit back and watch the results. Instead, you get a level that is set up to give you just a few different ways of arranging your walls and then you spend your time in one of two annoying modes. Played like a tower defense game you get to live the life of an office cleaner, vacuuming up and down the level to gain currency to spend on more towers. Played like a shooter, you can select several fun weapons to destroy bad guys and be constantly irritated by air units attacking you. The upgrades you can unlock are okay but don't add much to the game, and because you're so busy vacuuming or swatting flies you can't really observe the effects of your tower placements. So I don't feel it's a tower defense game, and the price is only justified by the very good graphics and physics - but why does a tower defense game need good graphics?
Love this game. After reading some lukewarm reviews maybe my expectations were low, but I found this to be a really well-balanced and satisfying game. The core combat experience is indeed a HOMM3 clone (even stealing some of the units from that game), but the creature types have interesting and varied enough abilities to make it work well. The RPG elements I think are fantastic, with multiple ways of progressing your character, spells, dragon's abilities, items, etc. without it seeming bewildering at the beginning. You are able to explore each at your own pace whenever they seem relevant and necessary. And the best thing for me is the travel around the map. Other reviews have pointed out that you can wander across units that are impossibly difficult to beat, but that is clearly intentional - in fact hovering over any unit gives you a very reliable rating of their strength relative to you (including "invincible"). But they are ALWAYS avoidable by backtracking and going a different way, which means you end up tentatively working your way round each island, seeking out a safe path and seeking enemies weak enough to beat, while you gradually improve your abilities. You then return to areas you couldn't survive in earlier to beat those previously invincible enemies. I think this is really clever - rather than a linear progression like most games, you get to see what you will face later right away, and choose your own path to how you will get there. It gives you visual goals to work towards, and lets you choose which to target at any point. It also makes the difficulty curve more transparent, so you don' suspect game is just adjusting enemy strength to whatever level you are currently at - because you can see their army size at an point and that it doesn't change. The difficulty curve was perfect for me, with enemies just tough enough and resources just scarce enough that it is always a challenge to progress, and always rewarding when you do. A rare gem.