One of the most famous sights in Old San Juan is El Morro, the huge Spanish fort at one end of the city that guards the mouth of the harbor. The Spanish began building it in 1539, but it is so massive it took them 250 years to complete it. (Declared a World Heritage Site by UNESCO in 1983.) Fascinating place, great views. The cannon are even still in place.
(Yes, that's Penny's nose at the bottom left.)