Complex sentence: Defeating Bowser the first two times earns Mario a key for opening another level of the castle, while the final battle releases Peach from the stained-glass window above the castle's entrance.
Split into simpler sentences: Defeating Bowser the first two times earns Mario a key for opening another level of the castle. After Mario defeats Bowser in the final battle, Peach is released from the stained-glass window above the castle's entrance.