This is El Morro, an old fortress in San Juan bay. I'm not sure how old this is but I believe it goes back to Spanish colonial days 100+ years ago. I thought it was interesting that this fort actually saw action during the Spanish-American War and was actually attacked by the United States! In one of these pictures you'll see a cannon still poised to fire on passing ships with a freighter offering a tempting target.
Seen from the fort:
