Photo: Castillo de San Marcos

Built in 1672 and renovated repeatedly throughout the years, St. Augustine, Florida's Castillo de San Marcos is one tough garrison ... sort of. Built from coquina, a shell-based "concrete" that can absorb cannonball fire like a Styrofoam absorbs a BB, the Castillo has never lost a battle. It's only been handed over by treaty. Today, it overlooks the city and makes for a great afternoon diversion. If enough people show up, they'll fire the cannon Friday through Sunday, on the half-hour from 10:30 to 3:30.