I just uploaded the interview that I did with Nasir Gebelli in 1998 at my company, Ion Storm, during an Apple II Reunion that I hosted. There's a lot of great information about his history in the industry and how he got started.
Nasir wrote games entirely in assembly language and did not save source code. He didn't even have a printer to look at his code! He wrote all his NES games on an Apple II and cross-developed them to the NES hardware. Even the SNES game, Secret of Mana, was coded on an Apple II, in 65816 assembly with no source.
I've been talking to Nasir recently about doing an in-depth interview. If you have any particular questions you'd like answered, please put them in the comments or email me at john at romero dot com.