Tower defense games have existed on Android since the very beginning. Remember Robo Defense? Yeah, me too. The genre has evolved over the years, and Kingdom Rush is one of the finest examples of it. Developer Ironhide Games has apparently decided to do Android gamers a solid and has made the original Kingdom Rush free. It was previously only a buck, but still. Yay, free stuff!
Kingdom Rush is an exquisite tower defense game with several ranged towers and barracks that train ground troops. You also have a hero unit that can roam the battlefield to shore up your defenses. As you advance through the game, you will unlock more advanced tower upgrades that can completely change the gameplay.
So what's the catch? There are in-app purchases. However, there have always been IAPs in Kingdom Rush, and I've never had an issue with it. The game is designed to be fair and winnable without paying anything. Kingdom Rush Frontiers continues to be $1.99 and Origins is $2.99.