That was surprisingly mellow music! It must have been peaceful to play.

There are all sorts of things which result in a piece sounding "mellow". One I use here is a very loose playing style, technically known as "rubato", where I don't hold exactly to the meter. Another thing it's got going in the "mellow" dept. is that I'm modulating between a G minor and C major key, with occasional asides in F major and Bflat major (although I did thrown in some Eflat major chords to spice things up). By not staying long in a major or minor key, you build a mood which is neither too cheerful, nor too morbid.

I could go on, and wax technical about some of the unique limitations of subtractive sythesis, buy I sense eyes glazing over...

When I'm playing, it's very focused, so I don't really get a sense of peacefulness from playing it. That comes afterward.