Complex sentence: Extra lives can be earned by freeing a certain number of prisoners scattered throughout the levels; some prisoners are trapped in barrels, or stuck in crystal cells, others are guarded by an enemy that must be slain in order to free them.
Split into simpler sentences: Extra lives can be earned by freeing a certain number of prisoners scattered throughout the levels. Some prisoners are trapped in barrels, or stuck in crystal cells; others are guarded by an enemy that must be slain in order to free them.