Charles V
For Earl Rosenthal, author of The Palace of Charles V in Granada (1985), the motto is closely associated with the Pillars of Hercules, which according to Greek mythology were built by Hercules, near the Straits of Gibraltar, marking the edge of the then known world. According to mythology the pillars bore the warning Nec plus ultra (also Non plus ultra, "nothing farther beyond"), serving as a warning to sailors and navigators to go no farther.
It is believed that the young Charles V adopted Plus Oultre as his motto at the suggestion of his doctor and personal advisor Luigi Marliano. The idea was to encourage him to ignore the ancient warning and encourage him to take risks, surpass himself and go "farther beyond".
It has also been interpreted as a phrase used to transcend nationality as Charles ruled over many different parts of Europe (particularly as being Holy Roman Emperor) and colonies all over the world.
Read more about this topic: Plus Ultra (motto)