All twelve notes, laid out like a piano — each alien falls down its own key's lane, black notes between the whites. Tap the matching key to fire. The first levels use the white keys; then a new sharp/flat joins every few levels — G♯, F♯, A♯, D♯, C♯ — spaced out to let your ear settle before the next. Aliens cloak into sonar blips as you climb, so you go by ear. Reference C plays before each level.