Many tower defense games are available for free on Google Play. But some are not really free.
You download the game, and thought you can play for free. But after a while, you realize that you can’t beat the game unless you buy some special items to power up.
This game is different. I downloaded the game, and even though there are purchases available, I don’t need them to beat the game.
You CAN beat the game if you play strategically: i.e. placing the right towers at the right spots.
And if you like the game, feel free to support them. Extra power up items can’t hurt!