La Puerta de Alcalá is one of the most famous monuments in Madrid. It was ordered to be built by the king Carlos III (Charles III) in 1774 and was designed by Francesco Sabatini. It is located in Plaza de la Independecia ("Independence Square"), near the city center, and in front of the main entrance to the Jardines del Buen Retiro (One of the biggest parks in Madrid). It is called Puerta de Alcalá ("Alcalá Gate") because it's placed on the way between Madrid and Alacalá de Henares.
It was built in neo-classical style, with Ionic chapiters on the columns. This monument consists in three semicircular arches and two rectangular arches in both sides. The incription in both east and west facades says: Rege Carolo III. Anno MDCCLXXVIII ("King Charles III, Year 1778). Over it there's a coat of arms held by the Fame and the Genius. At the beggining of its history it was used actually as a gate until 1869, when the Square was remodelled.

<small>(Source: es.Wikipedia.org)</small>