Agra Fort is an ancient fortress, located in the city of Agra in the Northwestern Provinces of India. The enormous complex, itself the size of a small city, was constructed by the Mughal dynasty in the 16th century on the site of an older fortification, and its massive red sandstone walls are one of the most notable of Agra's sights. During the Sepoy Mutiny of 1857, the fort was the center of British resistance in the city against the Indian rebels. The actions of Jonathan Small, a guard at the fort, during this incident would form the nucleus of one of Sherlock Holmes's investigations thirty years later, which Dr. John Watson chronicled in The Sign of the Four.