Upon his arrival to Spain, he continued with his reformation and modernization program and forged the Spanish national identity in the terms that we still currently know: under his reign both the national anthem and flag were adopted.
One of his major worries was the modernization of Madrid, so far an insalubrious town which had grown with little regard to planning and totally lacked of grand buildings and monuments. He built a proper sewage system, introduced public lighting and projected many of the buildings and institutions that are still today the city's landmarks, including the Prado Museum and the Alcala Gate.
For doing that, he counted on two of the architects who, thanks to their prolific work, have contributed to a greater extent to Madrid's current appearance. Juan de Villanueva, in particular, is considered as the best representative in Spain of the neoclassical style. He worked in several buildings in El Escorial, where he was highly influenced by Juan de Herrera's austere style, but he is mostly known for his works in Madrid, including the Prado Museum, the Academy of History, and the Astronomical Observatory. He is even responsible for the current look of the Plaza Mayor, which was rebuilt after a fire in 1790. His project included the monumental arcades that surround the precinct, the homogenization of the height of the buildings and the picturesque Knivers Arch. In these works, also Francesco Sabatini collaborated. Born in Italy, where he became the King's favorite architect, he was brought to Spain to complete the works of the Royal Palace and remained there for the rest of his life, working in many of Madrid's most significant buildings, where he showed a deep influence by the works of the Italian Renaissance architects. Apart from his interventions in the Royal Palace and the Plaza Mayor, he is particularly famous for the Alcala Gate, the city's landmark, but he also planned the city's sewage system, built the Royal Customs House and completed the Saint Francis Basilica, one of the grandest temples in the city.

