Props to 1UP for getting an interview with this guy, a living legend. You already know many of his songs from Contra, Castlevania III, and TMNT. What I didn't know before reading was that he designed the VRC6, a piggyback chip included in Castlevania III (Japanese version only) which added 3 more sound channels. There's a youtube video here if you want to hear a comparison.
He mentions the famous Konami Sound and how he helped develop it. Data suggests he could be the father of the single-channel echo, basically a technique to fill up gaps between melody notes with quieter elements to simulate delays and reverb without using the second channel.
I am still dying to know what tools were available to make these songs. Right now I am picturing the soundtrack as a catalog of punch cards.