The old fort dates from 1672 and is constructed from locally quarried coquina rock which is a sediment composed of marine invertebrates or shells. It has been used as a building stone in Florida for over 400 years. The stone makes a very good material for forts, particularly those built during the period of heavy cannon use. Because of coquina's softness, cannon balls would sink into, rather than shatter or puncture, the walls of the Castillo de San Marcos.