The heart of Antigua, and one of the finest colonial squares in Central America. At its centre stands the Fuente de las Sirenas (Fountain of the Sirens), designed in 1739 by Miguel Porras — the sinuous mermaid figures have become a symbol of the city. The fountain wasn't purely ornamental: it served as a water source for households without private supply well into the 20th century.
On the north side of the plaza, the City Hall building contains a small museum of colonial weapons, period dress, and artwork from the Spanish colonial era. It's open daily 9am–noon and 2–6pm, and free to enter. The courtyard itself, with its colonial fountain and stone arches, is worth seeing even if you skip the exhibits.