I found this gem on Steam the other day. It's a Tower Defense mixed with RPG. It runs on Linux natively and is a true time robber and addictive. You got the traditional Tower Defense meanwhile you got Heroes that can level up and learn new things and deployable armies to help you out.
Description: "Command your elven army and defend mystical lands from sea serpents, evil sorcerers, and wave after wave of gnoll tribesman, all with the help of brand-spankin’ new towers, heroes, and spells to fend off every last baddie."
My 12 min. video of the game:
The game is on Steam: