The complex was built by the Spaniards between the end of the sixteenth and the beginning of the seventeenth century, after Porto Santo Stefano became part of the State of the Presidi. The place chosen for the construction of the defensive fortified structure had already been chosen by the Sienese for the construction of the tower of Santo Stefano during the fifteenth century.
Some building materials were used from the pre-existing tower after its demolition ordered by the Spaniards because it was not considered up to the most efficient coastal defensive system under construction. However, the construction work on the defensive complex continued very slowly, ending only in 1636 with some improvements made after the development of a subsequent improvement project by the military engineer Pedro Álvarez. For centuries the fortification functioned effectively for its defensive functions, succeeding in various circumstances in resisting and repelling enemy attacks and incursions from the sea.
At the beginning of the nineteenth century the French reinforced the defensive structure to resist possible attacks by British ships during the Napoleonic period. Later, the entire territory became part of the Grand Duchy of Tuscany and the Lorraine carried out some restoration work on the floors. After the unification of Italy, the fortress continued to perform the military functions becoming a strategic garrison during the First World War, while in the post-war period of the Second World War the two buildings were added on the highest terrace to temporarily house the municipal offices as a result of heavy bombing that had affected the entire town.
A series of restorations carried out in the second half of the twentieth century made it possible to bring the whole complex back to its former glory, and since 1997 it has been used as a museum complex with the inauguration of exhibitions on topics of the territory.
The Spanish fortress of Porto Santo Stefano presents itself as an imposing complex that develops in a quadrangular plan, with imposing shoe bases that in the upper part culminate with a crowning of protruding corbels that enclose as many drains, where the parapet that borders the terraces summitali: this crown recalls in the stylistic elements what characterizes also the Buranaccio tower.
The fortification consists of two buildings set against each other, with the smaller one that is articulated on the side facing the sea.
The external walls have some sections covered in stone (mainly at the corners) and others in whitewashed plaster, with numerous loopholes that open at various heights. Access to the complex is on the side facing the ground, where an entrance door opens to the upper level, preceded by a long and characteristic flight of external stairs that ends with a bridge that has replaced the lost original wooden drawbridge. A series of walkways, protected by curtain walls, connect the various parts of the fortress, including a series of flights of covered stairs leading to the highest terrace, where the two buildings were built to post the seat and the municipal offices after World War II. Originally, the top part was characterized by the presence of a series of unfortunately lost boxes.
In the inner part corresponding to the basement there were tanks that guaranteed large deposits of water, while at the upper levels there were the spaces used for the lodgings of the sentinels.