listening over to different versions[1] and it is amazing how different the game sounded on different systems (sometimes with completely different soundtrackks). The Apple II sounds like it uses two square waves only. While Amiga, DOS, PC-98, Sharp x68000 and Sega Genesis sound like FM synth, FM Towns and SNES sound like wavetable synth, Gameboy is two squares, NES is two squares and a triangle plus noise.

[1] https://www.youtube.com/watch?v=0q_6tOpnquI