This splendid monumental gate commissioned by Carlos III is the most notable sight of the Independence Square. It was built in 1778 and it represents the Neo-classical style.
If you think this gate resembles the Arc de Triomphe in Paris and Brandenburg Gate in Berlin, you are right. Alcalá Gate is similar to these gates in structure and it was the first modern triumphal arch built in Europe.
Fortunately, it was not extensively damaged during the civil war, but it was never fully repaired, so you can still spot the marks.
There is also a famous Spanish song inspired by this monument, titled La Puerta de Alcalá.
Wikipedia https://en.wikipedia.org/wiki/Puerta_de_Alcalá
Address Plaza de la Independencia 1, 28001 Madrid, Spain
Coordinates 40°25'11.95" N -3°41'19.409" E