Flare a free, open-source RPG game
Flare is an open source, 2D action RPG licensed under the GPL3 license. Its game play can be likened to the games in the Diablo series.
While the core engine is written in C++ with SDL2, there is an easy to use modding interface. Using the modding interface, it’s possible to create entirely new games.” source: flarerpg.org
You can download the game from the project webpage, sourceforge, github or install it directly from your distribution repository through package manager.

Zombie Defense

A modern hybrid of the classic RTS and modern Tower Defense.
 Recruit soldiers, place them on slots and let them shoot incoming waves of zombies by their own.
 Order supplies, throw grenades, order airstrikes and many more… Perfect mix of strategy, tactics and micromanagement.” source: steam description

 You can download the game client from the project webpage or steam.