If you're into tower defense games, Anuto TD on Android via F-Droid is a solid FOSS one. Has this charming hand-drawn look to it as well and a number of maps to play with.
Also, and this may just be me, but I really enjoy that it's played in portrait/vertical view instead of landscape/horizontal. Dunno why but I find handling phones in landscape awkward feeling, personally.
For those interested, as mentioned in the first post, you can download Anuto TD for free via F-Droid. Here's a link, which if you've F-Droid or Droidify installed should prompt you to open via them, I think.