I recently went to Puerto Rico for work and ended up staying there 12 days (was only supposed to go for 5). I spent most of my time at the power plant, but last Saturday (8/21/10) we had a day off and were able to go into Old San Juan and take a tour of the Spanish fortress "El Morro." The fortress was originally started in 1538, and expanded upon numerous times, with walls surrounding most of the old city and another fortress at the eastern end of the old city.
San Juan was an extremely important port for the Spanish, and the level of fortification there shows it. The fortress was attacked several times by the English during Spanish occupation, and was only taken once in a land invasion by George Clifford in 1598, who later abandoned it after all his troops got sick (I can't imagine why, the humidity was 70%+ every day I was there and it was crazy hot). The place was very impressive, and if you ever get a chance to go to Puerto Rico, I would highly recommend it. Unfortunately, the only camera I had was my cell phone, which took decent pictures, so I'm sure you could find better ones online somewhere, but if you want to check them out, here's the link. There are a couple of other random pictures in there of where I stayed and the plant I was at, just ignore those.
Link to Flickr