Juan de Garay founded the historic core of Buenos Aires, the Plaza de Mayo, upon the city's second founding in 1580. The plaza's prominent buildings create an architectural timeline: the Cabildo, or Old City Hall, and Metropolitan Cathedral are vestiges of the colonial period (18th and early 19th c.), while the Pirámide de Mayo (Pyramid of May) and the buildings of the national and local government and union offices reflect the styles of the late 19th and early to mid-20th centuries, including some that typify the severe Fascist style popular in South America at the time, with smooth surfaces and enormous Roman-style metal doors. In the center of the plaza, you'll find palm trees, fountains, and benches. Though many of these facilities are in need of an upgrade, the plaza is still full of local people at lunchtime, chatting and eating takeout food.
Plaza de Mayo remains the political heart of Buenos Aires, serving as a forum for protests with many camping out here overnight, including a group of Islas Malvinas veterans denied combat pay who have been living in a tent since 2008. The mothers of the desaparecidos, victims of the military dictatorship's campaign against leftists, known as the Dirty War, have demonstrated here since 1977. An absolute must-see for understanding Argentina's recent history, you can watch them march, speak, and set up information booths every Thursday afternoon at 3:30pm.
Mass demonstrations are very common here, and most protests begin in front of the Casa Rosada (now separated from the crowds by permanent barricades) and proceed up Avenida de Mayo toward Congreso. For the most part, these demonstrations are peaceful, usually led by people who have suffered the economic consequences of the peso crisis, known as piqueteros. However, at times, protests have broken into violence, so be aware when demonstrations are occurring and leave immediately if things seem to be getting out of hand. The presence of a group of protestors with covered faces carrying metal sticks is a definite sign to leave.