Libretro is a simple but powerful development interface that allows
for the easy creation of emulators, games and multimedia applications
that can plug straight into any libretro-compatible frontend. This
development interface is open to others so that they can run these
pluggable emulator and game cores also in their own programs or
devices.

NXEngine is a Cave Story game engine clone developed by Caitlin
Shaw.

Cave Story is a platform-adventure video game developed over five
years by Daisuke "Pixel" Amaya. Cave Story features 2D platform
mechanics and is reminiscent of the classic games the developer
played in his youth, such as Metroid and Castlevania. It received
wide critical acclaim for its compelling characters, setting, story
and gameplay.

Homepage:
https://docs.libretro.com/library/nxengine/
