The conservation of the fortification in Tarragona was not an easy task. In 1884 the Roman wall was declared a National Monument, and in 1966 it was declared a historical and artistic complex. These measures distant in time did not achieve a joint protection of it. This study deals with the transformation of the bastion of Cadenas in the San Antonio Promenade. It is an example of urban speculation during Franco’s developmentalism. Our contribution is the result of an analysis and comparison of various types of documentation: military, engineering command headquarters as well as municipal and state. We demonstrate that the destruction of the bastions was due to the low value given to these defensive structures, which were considered an obstacle to urban expansion and development.