St. Augustine, Florida vies with Pensacola for the title of "Oldest City." I haven't paid much attention, aside from reading the occasional bumper sticker.
Who cares about that when there are forts to look at? St. Augustine has America's oldest masonry fort, the Castillo de San Marcos, completed by the Spanish in 1695.
The National Park Service maintains this beautiful place. It's not as large or elaborate as the "Third System" French-style forts the US started building between the War of 1812 and its Civil War, but it's every bit as formidable in its defense. Where most American Third System forts were built using common clay brick contracted from various producers, the Castillo was constructed with locally-quarried coquina, a soft sedimentary limestone made up of shells. This stone is soft enough that cannon balls would sink into the surface; brick would shatter or crumble when struck.
I got lucky on this photo; it's one of only 4 that survived a hard drive crash that wiped out everything I had in February of 2010, but it's also the most dramatic shot. There was a thunderstorm building behind me and the clouds overhead made for a perfect "When your jaw drops, trip the shutter!" moment. Within 5 minutes, the rain was coming down by the bucketload.