Something cool about Old San Juan is they have a fort! Apparently it's a National Park... I had no idea, it wasn't on my list, maybe I only have the nature national park list. Another fun fact... a scene of Pirates of the Caribbean was filmed here. Not sure when of the 4 or 5 of the movies... but I watched the scene on YouTube and it was cool... and cool that I have been there!
I'll let Wikipidea explain more about the forts...
Lying on the northwestern-most point of the islet of Old San Juan, Castillo San Felipe del Morro is named in honor of King Philip II of Spain. The fortification, also referred to as el Morro or 'the promontory,' was designed to guard the entrance to the San Juan Bay, and defend the Spanish colonial port city of San Juan from seaborne enemies.
In 1983, the citadel was declared a World Heritage Site by the United Nations in conjunction with the San Juan National Historic Site. Over two million visitors a year explore the castillo, making it one of Puerto Rico's leading tourist attractions. Facing the structure, on the opposite side of the bay, a smaller fortification known as El Cañuelocomplemented the castillo's defense of the entrance to the bay.