Plaza de Mayo is the most important square in Buenos Aires. This is where the Spanish colony was founded by a conquistador in 1580. By the end of the 16th century, the Royal Fort of Juan Baltasar of Austria was built here. El Fuerte was the residence of the Viceroys of the Río de la Plata from 1776 until the May Revolution. On May 25, 1810, a new local government called Primera Junta was established and Cornelio Saavedra was appointed as the first president. In short, you are standing at the birthplace of the city, the location of the first attempt at self-government and the origin for the Argentine War of Independence leading to final freedom from Spain.
