The art style is FANTASTIC. That is the only positive game element to be had here. Save your money and stay away, regardless of the pandering reviews from what I can only assume are people who have never played tower defense games before. I mean hell, if it wasn't for the art style this thing wouldn't even survive on Miniclip.
The game feels like a continuing list of bad design choices, a case study in what not to do in a tower defense time. When I complain I feel like a lecturer in basic game design: "encourage strategic play", "present consistent rules for your game world", or maybe "release without game crippling bugs and advancement locks". It's bad enough now, but be greatful you weren't playing when the campaign tech tree was locked and you'd be forced to menu-delete a game when an armored enemy type showed up an you had no units capable of damaging it.
Almost every mistake (as I'm coming to call them, because I honestly don't think any of them were actually thought through properly), is at once trivially annoying and long term game breaking. Manually resetting and time consumingly repicking your tech tree to deal with new enemy types every level may seem simply irritating, but gradually emerges into an overall expectation that you pick the "right" tech tree that enables victory, and all others will fail by default. Having to painfully micromanage your reloads is grating, and eventually ensures that every strategy will be limited to your clicktimes and not your strategy, dooming any kind of open-world defense. And the rediculous armor and repulsion systems vex you piece by piece as your weapons and equipment become obsolete, before finally running the final, game destroying flaw: you will not lose when your defenses are overwhelmed, you will lose when the charming little boxlike enemies have countered every possible defense and your entire list of defensive options have become obsolete.
It's like the equiptment they give you is just a taunt of what your strategy could have been: oh hey, here's a rapid fire turret. Too bad it does no damage to enemies after level 4, when they all have armor. Hey, check out these repulsion buildings to herd the enemy into chokepoints. Sucks that every other enemy after level 6 has become "intelligent" and will simply run up and eat them. Laser weapons? Armor reflects them - useless. Slowing razor field units? Can't be used with the anti-armor explosive guns, which destroy all your own buildings, including your own defenses, because apparently future scientists drink too much coffee to understand friend-foe ID. Similarly, saving up money in the campaign feels rewarding until you find out that the game raises the difficulty with your cash level and thus it's advantagous to avoid any kind of strategic spending. RotT plays like a cod game, where every interesting idea gets thrown out within ten minutes because it happens to be too fun and interesting to keep using.
I like playing tower defense games because of the evolution of your goals and the resource management involved. I'm not sure I like playing them when resource management is a menial clusterf*** and the evolution of methods is only happening in the enemy forces. I've never had a tower defense game where I've felt so much like *I* was the computer, and the enemy forces were actually the human, getting to play adaptively against me with all the strategy and choice-making I was missing. Playing like a bot isn't a good feeling. I do not recommend playing like a bot. I do not recommend this game.