The main operation on Mullet Key, however, became Fort De Soto in 1900, named for Spanish explorer Hernando de Soto. The Army post was officially a subpost of Fort Dade, which was constructed on Egmont Key. These posts were to contain batteries of artillery and mortars to protect Tampa Bay from any invading forces. Construction of Fort De Soto began in November 1898 and completed in 1906. The foundation was constructed of a seashell concrete formula, and the walls and ceiling used a seashell, stone, and concrete mix. The main attractions at the completed post were the artillery and 12-inch coast defense mortar batteries, Batteries Laidley and Bigelow. The post consisted of 29 buildings. The post's features included a large barracks, a hospital, a guardhouse, a blacksmith and carpenter shop, an administration building, and mess hall with kitchen, a bakery, and a storehouse. The site also had brick roads, concrete sidewalks, and a narrow-gauge railroad to aid in moving materials and supplies around the post.