C-Dogs SDL is a 1 to 4 player overhead run-and-gun game. The player character(s) are elite soldiers, taking on mission after mission to defeat evil in any form. That is, you kill it or blow it up. Sounds violent? Yep, that's right. Still, the emphasis is on gameplay rather than gore. But the game will involve a lot of mowing down of enemies—and the occasional unlucky civilian—so now you have been warned.
Yes, the game is indeed not free software / open source because some media files are under copyright restrictions. However, it is “almost open source”, the code certainly is, and most of the data are as well. The developer tries to replace those files in the long run: https://github.com/cxong/cdogs-sdl/issues/336
EDIT: Version 0.6.0 is now free software.