The land that is now Ft. DeSoto and Egmont Key was declared a military reservation after an 1849 survey done by the Army Corps of Engineers. This survey group, which included Captain Robert E. Lee, spent about a month on Egmont Key and in the surrounding area looking for potential sites for coastal fortifications.
The need for permanent fortifications was decided by the hostilities with Spain during the Spanish American War. Tampa was a staging embarkation embarcation point for troops going south to Cuba to fight. It was feared by the military that the Spanish Navy might attack the port of Tampa.
Temporary artillery installations were placed at Egmont Key, Passage Key and Mullet Key until plans could be completed for permanent structures. The war ended before permanent fortifications could be completed but it was deemed necessary to add Fort Dade (Egmont Key) and Fort DeSoto (Mullet Key) to the U. S. Coastal Defense network.
The fortifications on Egmont Key include numerous gun batteries, a mine storage building and the Guard House, the post jail that has been fully restored.