Photo: Castillo de San Marcos drawbridge

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.