Out of everything Defenders did, their complete control of the map feels the most right. Just by taking your two fingers and turning them on the screen, the map will turn a complete 180 degrees. I'm bringing this up at the start as a bit of a warning. Playing other games afterwards -- that do not offer the same kind of map control -- will just feel off.
There is a learning curve. As you find out what each ability does the game becomes progressively harder. While you have the option to pay, you don't need to. The game is supported by ads, but you can also play offline -- without affecting the gameplay at all -- in order to turn them off. By playing different levels to try and get the highest score, you will end up being able to unlock all of the towers in the game.
With a story line and with boss fights, Defenders isn't so easy that it makes you uninterested in playing. Neither is it so hard that you will need to go online and look for guides. Instead, it rests in this Goldilocks zone of not too warm and not too cold.
My issue with Defenders is that it is pretty CPU intensive for the phone. This means that in terms of playing, it will drain the phone pretty quickly. Also, when you do beat all the levels and finish the achievements, that's it. But all things considered, this game should be in your top 10.
Still, if there is a game that can beat this Defenders: TD Origins, it's our number one.