In 1580, Juan de Garay reestablished Pedro de Mendoza’s failed settlement on what is now the Plaza de Mayo , surrounded by most major national institutions. The barrio’s axis is Avenida de Mayo, linking the Casa Rosada  presidential palace (1873–1898) with the Congreso Nacional  (National Congress, 1906); the broad perpendicular Avenida 9 de Julio splits Monserrat in half.
At the northeast corner, renowned architect Alejandro Bustillo designed the Banco de la Nación (1939). Immediately south of the Casa Rosada, the marble facade of the Ministerio de Economía (Economy Ministry) still bears pockmarks from strafing naval planes during 1955’s Revolución Libertadora that sent Juan Domingo Perón into exile.
One of Avenida de Mayo’s literal landmarks is Mario Palanti’s Palacio Barolo (1923), an office building topped by a rotating semaphore visible from Montevideo’s Palacio Salvo (the work of the same architect). In 1923, when Argentine heavyweight Luis Angel Firpo fought Jack Dempsey in New York, the Barolo erroneously announced Firpo’s victory with a green light from the tower.
South of the Plaza de Mayo, most landmarks are modified colonial buildings. From the roof of the Casa de la Defensa, porteños poured boiling oil on British invaders in 1806–1807. The building now houses Télam, the official government press agency.