At the mouth of the Tagus River and east of the main beach is the fortification of Fort of São Julião da Barra. It was used to protect the Tagus from enemy ships, but in the 20th century it became the official summer residence of the Minister of Defense.
In the 19th and 20th centuries, Carcavelos was an important landing point in the international telegraph network, providing crucial communication links for the British Empire; most of the links were run by Cable & Wireless company forerunners.[2]