The original developer Milton Guasti worked on the game for 10 years. It's written in GameMaker Studio. In 2016, Nintendo did a DMCA takedown of the project, before releasing their own remake 'Metroid: Samus Returns' in 2017. From the original binary, the source code was reverse engineered by a group of fans in the AM2Rrc project. Since then, a group of developers have been working on bugfixes and improvements. I'm playing this community version, currenly on 1.5.
This unofficial Metroid II remake looks, feels, and sounds great. I really enjoy the ledge grab move, very useful.
After finishing the original, I felt like going back to this great remake of the second Metroid game. I played for a few minutes this morning and got a bunch of progress. This game is easy compared to the original.