It's not exactly tower defence, although i think it's close, but i am currently recovering from a pretty serious Bad North obsession:
https://www.badnorth.com/
You defend procedurally generated islands from viking hordes, but rather than towers, you use up to four squads of soldiers from an army you grow over dozens of islands. The art and sound design are lovely, and the gameplay is engrossing. Each island takes a few minutes to play, and a campaign lasts a few hours.
It's something like 5 dollars on iOS and Android, and has no ads or IAP.
