The source code for a bunch of retro games ranging from the NES, Genesis and Sega CD is currently available at GitHub.
The source comes from programming veteran, Chris Shrighley, who has more than 30 years in the industry, having worked with companies like Eurocom developing games for the NES, and more popular ones like Disney Interactive, working on games for the Genesis. He also worked on the Sega CD version of Batman Returns.

The list of games that had source code released by him:

• Magician (NES)
• John Smith Special Agent (aka James Bond Jr.) (NES)
  
• Hero Quest (previously unreleased NES game)
• Ex-Mutants (Genesis)
• Cliff Hanger (Genesis)
• Batman Returns (Sega CD)
• Gargoyles (Genesis)
• Gargoyles Protype/Test (SNES)

I’ve been a games programmer for a very long time now, and over the years I’ve collected quite a lot of source code from the games I’ve worked on. Unfortunately, a lot of the code is still owned by other people, and cannot be shared. I have older archives though, on the NES, and Sega Genesis, that I can share, and they are linked below.

Feel free to download and play with the code as you will. One important thing to note, this code is made available for educational purposes only! You are NOT allowed to make money from this source code, in any way, shape or form.

Cheers!

While the release is nowhere recent, it's always nice to see more and more developers that consider releasing the source code as open source and available to the public for others to delve into.

Source

 

[Kagami-san]

Generally decompiled code is not the same thing as original source code, either.

It depends and I suspect that something older would be somewhat more readable but certainly trying to disassemble anything back to a high level programming language is difficult even with tools.

Really need a strong knowledge of assembly / x86 (for Windows apps anyway)