If you're looking for something new in the stagnating world of tower defense, this is it. City Conquest turns the genre on its head, by forcing players to defend their own territory and attack others simultaneously, with a combination of real-time and turn-based gameplay. It's a bit hard to wrap your head around, but trust me, the experience is well worth the effort. City Conquest is a free download in the Play Store (ad-supported, no in-app purchases) for devices running Android 4.0 or later with a resolution of at least 960x720.
Like the RTS games of yore, each round starts with a Capitol (town center/nexus/construction yard) for both players.