King Charles I of Spain, also known as Charles V, Holy Roman Emperor, ruled from 1516 to 1556. He was born in 1500 and was the grandson of Isabella I of Castile and Ferdinand II of Aragon. His reign marked the height of the Spanish Empire, as he inherited vast territories across Europe and the Americas. Charles I was known for his efforts to maintain Catholicism and combat the Protestant Reformation. He faced numerous challenges, including wars with France and conflicts with the Ottoman Empire. Ultimately, he abdicated in 1556, dividing his empire between his son, Philip II of Spain, and his brother, Ferdinand I.