Defense Grid 2 HD is a more realistic tower-defense game, with a modern military theme. In comparison to its cutesy competition, it certainly has more grit, but it also has excellent gameplay.
There is a lot of tactical depth to the game, which may make it difficult to get into if you've never played a tower defense game before. However, I suspect that genre veterans will relish the challenge it presents.
Defense Zone 2 HD costs US$2.99 for the full version, but there is a light version available if you want to check that first. It’s definitely one of the best tower defense games on Android.