If you are looking for a tower defense game with a more retro feel, you should check out Tower Defense eXtreme. Think back to the games you might have played in the 2000's, with simple graphics and an image pasted on the background. With a free-roaming map, you build the direction you want with straight forward skills. There are four tower types that upgrade linearly. You have the low power cannons, area of effect slows, the heavy splash damage cannons, and the fast rocket attacks.
In some levels, the entrance and exit are just one tower away from each other, whereas on other maps there is a looping path. If you are in trouble or need time to think between waves, you can pause the game to build or destroy towers. But be careful, with the partial refunds it might be better to just restart the level.
TDX is a game that sells itself as something without any gimmicks, and delivers an easy offline play. But what it does not deliver is an easy learning curve. With almost no description of what each tower does, or what the upgrade paths are, you will need to test things out.
But maybe one of the best tower defense games is at number 2 for free-to-play android users, so check out the next slide!