Complex sentence: The initial game is divided into six episodes (with three more episodes added subsequently; see releases section), each of which contains three planets (worlds), with every planet itself consisting of two levels (some planets have an additional secret level).
Split into simpler sentences: The initial game is divided into six episodes (with three more episodes added subsequently; see releases section). Each episode has three planets (worlds), with every planet itself consisting of two levels (some planets have an additional secret level).