St. Thomas is the gateway isle of the U.S. Virgin Islands in the Caribbean. It's known for its beaches and snorkeling spots. Territorial capital Charlotte Amalie, founded by the Danish in the 1600s, is a busy cruise-ship port. Historic buildings include a 1679 watchtower called Blackbeard’s Castle, in reference to the area’s pirate history. On the harbor, 17th-century Fort Christian is now a local-history museum.