Cross-Lib is a C a set of tools and library that provides a retro-hardware abstraction layer to code "universal" games with simple graphics and sounds (whenever available on the target hardware) for *hundreds* of different vintage systems from the '70s till the '90 (computers, consoles, hand-held consoles, scientific calculators, hacked toy computers, etc...):

https://github.com/Fabrizio-Caruso/CROSS-LIB/blob/master/docs/STATUS.md

I have already written 3 universal games with Cross-Lib:
- Cross Chase (somehow a sort of real-time version of Gnome Robots and probably world record holder for the game with the highest number of "ports", i.e, more than 200)
- Cross Shoot (somehow similar to Robotron. It is largest game so far with many game elements, enemy types, items and secrets)
- Cross Bomber (mini-game clone of Air Attack, aka Blitz)