Old San Juan was definitely a highlight of my Puerto Rico trip. It’s a beautiful area with a very European charm. I’d like to live there for a few months to really give it a good thorough exploration!
This is a 465-year-old neighborhood originally conceived as a military stronghold. Its 7-square-block area has evolved into a charming residential and commercial district. The streets here are paved with cobbles of adoquine, a blue stone cast from furnace slag; they were brought over a ballast on Spanish ships and time and moisture have lent them their characteristic color. The city includes more than 400 carefully restored 16th- and 17th-century Spanish colonial buildings.
— From http://www.topuertorico.org/city/sanjuan.shtml