In the mid 19th century, military architecture experimented a rapid metamorphosis as a result of armament advances. In a few decades, an architecture that had replaced the traditional bastion systems would become obsolete. Nevertheless, between the decline of bastion fortresses in favour of armoured ones from the beginning to the middle of the 19th century, it is possible to identify a few construction modus operandi that characterizes the coastal batteries of that period. Researching into thefortresses of Cartagena allowed us to see how the international fortification theories were applied within the peninsular context. In the specific case of the 19th century coastal batteries, it is extremely interesting to study the project and constructive processes of the Fuerte Fusilero de Navidad in order to attain knowledge from debates, fortification theories and techniques, and to draw conclusions about the characteristics of a coastal battery before the introduction of armours.