The game is a sort of tower defense game...but also, it's not really that at all. Okay, so yes, you have to defend a village from oncoming enemies, and in order to do so you can build towers. Beyond that, though, it is worlds away from your typical tower defense. Rounds begin by giving you some time to explore your area, harvesting Scruffles (which essentially give your village more hitpoints), mining for ores to bolster your defenses, and looking for things to sell to get money for your towers. Then you have to scurry back to your village to cash everything in before the raid starts.
That's where the game gets even more interesting and genre-defying. The incoming Grocks are huge, but when you come into contact with one, the game switches from its normal overworld to a battle arena with several small Grocks that you battle. When they're defeated, you go back to the overworld to continue defending your village.
So at first it's a pretty disorienting game. I struggled for a bit to figure out what game I was playing. It became much more enjoyable once I realized that it was just different. Although, in the end I think I wish it was either more tower defense, or more real-time combat, or...I don't know, it just sort of felt like it was hovering awkwardly.