I definitely get the Cave Story vibe, but for some reason they don't really scream "chiptune" to me. It doesn't really sound like you're following any particular system's particular specs or restraints, so I can't really give you advice on the matter.
If it's a Cave Story vibe you're going for you've pretty much nailed it. If you want a more general chiptune sound, I recommend sticking to square/pulse waves, avoiding full chords, avoiding overlap of new notes over decaying notes, and using arpeggiated chords where applicable.