The current castle was built in 1651 at the mouth of Lake Izabal by order of the Oidor Antonio De Lara y Mogrovejo, who gave it its current name in honor of the king. The Río Dulce connects Lake Izabal to the Caribbean Sea and was subject to frequent pirate attacks in the 16th and even into the 18th centuries (Wikipedia). Admission 75 quetzales, open until 5pm. There is also a bathing beach, a small pool and showers/changing rooms, so pack your bathing suit.