One of the best Tower Defense games to ever hit android was Sentinel 3: Homeworld from developers Origin 8 Technologies. It had awesome graphics and plenty of content for you to play through, with just the right amount of challenge and there was lots to learn, so you couldn't just plow through the motions of guided play. Origin 8 is back with their next game in the series, Sentinel 4: Dark Star, the long awaited sequel to Sentinel 3. Dark Star has all the fondness of its predecessor, but with even more challenge and more weapons, units, and defenses to set up and protect yourself from the impending attack waves.
The campaign is big and grand, and spans across 26 unique maps so you'll not only have lots of story content to play through, but you'll be traveling through a distant world while doing it. Origin 8 states that in the campaign you'll be defending your territory across multi-screen landscapes, so expect to panning and zooming quite a bit here. Truth be told you don't really have to zoom, but I find it makes for a nicer set of visuals sometimes when you can see the action of what's happening close up. There's also an endless mode for never ending waves of onslaughts to see how long you can survive.
The gameplay has gotten a nice little upgrade, and so have the visuals, as there is now real-time dynamic lighting and shader effects included to give things a gorgeous look and bring some atmosphere and immersion to the alien landscape that you'll be fighting on. Like with all tower defense games, after you defeat enemies on any map you'll get a little bit of currency rewarded, which you can use to buy new units and upgrade existing ones. There's also an element involved to push you to complete stuff with the highest score you possibly can, as you can upload that score to something that Origin 8 is calling the Global Nexus which helps contribute to a worldwide co-op initiative that earns players rewards. During each map, you'll be able to unleash devastating attacks from drones and control commander heroes to help back you up in the fight, and of course call in support from the Sentinel ship weapons which do massive, massive damage. Sentinel 4: Dark Star will set you back $2.99, and you can grab the game from the Play Store link.