Please answer the following question: Blackbeard's Castle - Blackbeard's Castle is one of five National Historic Landmarks in the U.S. Virgin Islands. It is located in the city of Charlotte Amalie on the island of St. Thomas. Built in 1679 by the Danes as a watchtower to protect the harbor as well as Fort Christian. Blackbeard's Castle was originally called Skytsborg Tower (meaning sky tower). It is located at the highest point on Government Hill. Skytsborg Tower served as a very effective vantage point for Danish soldiers to spot enemy ships. Given a choice of categories company, educational institution, artist, athlete, office holder, mean of transportation, building, natural place, village, animal, plant, album, film or written work, the text refers to which one?
A:
Building