Prospero uses Ariel's magic to safeguard Gonzalo and the King of Naples. The two men are constantly being plotted against by Antonio and Sebastian, who want to kill them. For example when Antonio and Sebastian try to stab Gonzalo and the King of Naples in their sleep, Ariel intervenes by shouting and awakening the sleepers, leaving Antonio and Sebastian to concoct a lame story about protecting them from a lion.
Later, Sebastian and Antonio are still at it. When Gonzalo and the King of Naples get tired, Sebastian and Antonio plot to kill them that night. Instead, they get whisked to a magic banquet where Ariel makes all the food vanish.
Interestingly, it is not dangers from the island that threaten these two men as much as their fellow travelers from so-called civilization. All in all, it is Prospero and Ariel's magic that safeguards Gonzalo and the King of Naples.