The Tondo (as it is called by the local population) is located at the Colle del Pero hill. It is a rare example of an ancient roman Ludus dated to the 1st century AD, a sort of a little amphitheatre used as a gym or training ground for gladiators (see List of Roman amphitheatres).
The Palazzo Rospigliosi, in the city center, occupies the site of an ancient medieval castle, which is first referenced to when Pope Paschal II destroyed Zagarolo in early 12th century after the rebellion of the Colonna family, of which the palace was one of the strongholds. For many centuries it remained under the Colonna influence, and was besieged and destroyed several times due to the rivalry between the papacy and the Colonna family. In the 16th century the Colonnas and the papacy started having peaceful relationships, and Zagarolo became a duchy, hence the Palace became known as Palazzo Ducale. The palace now was used as a residence and two major wings were added to it towards the piazza, frescoes were painted and a hanging garden was built. In 1591 a council of eight cardinals was held in the Palace, with the mission to revise the Bible for a printed edition, among them were Marcantonio Colonna (the cardinal) and Saint Robert Bellarmine. The building is characterized by frescoes painted by manierist artists of 16th century, attributed to Dutch painters, to Antonio Tempesta, and to the Zuccaris (Taddeo Zuccari and Federico Zuccari). Here in 1606 Caravaggio created masterpieces for the Colonna family, in exchange of refuge on his journey towards Naples, between Zagarolo and Paliano he executed the Supper at Emmaus, Mary Magdalen in Ecstasy and perhaps Saint Francis in Prayer. In the Palace also worked Carlo Maratta and Ludovico Gimignani, who died in this palace in 1697. Later the Palace became property of the Pallavicini family, until it was sold to the comune in 1979. During World War II the palace was transformed in a German military hospital. Today the Library of Zagarolo and the Toy Museum are inside the palace, and it is used for conventions, exhibitions and other cultural events.
Church of St. Lawrence Martyr (1607) (Chiesa di San Lorenzo Martire), located in Piazza del Risorgimento, it is the church dedicated to the patron saint of Zagarolo St. Lawrence of Rome.
Convent and Church of St. Mary of the Graces (Santa Maria delle Grazie), located in Piazza Santa Maria
Church of St. Annunziata (1580–1582) (Chiesa di Santissima Annunziata), has a peculiar octagonal belltower, and dominates the skyline of Zagarolo.
Church of St. Peter (1717–1722) (Duomo di San Pietro Apostolo), a Baroque church built on the site of a more ancient church. It has an elliptical dome 46 meters high.
The first gate to be encountered entering the city center from the southeast is Porta Rospigliosi, decorated with bas-reliefs in the 16th century and Roman busts and theatrical masques. The other gate, at the northwest entrance to the city is Porta San Martino, named in honour of Pope Martin V (Oddone Colonna).
The Zagarolo Toy Museum, located in Rospigliosi Palace. The Museum offers a social reconstruction of toys and the act of playing in a series of sections that, through a historic excursus, crosses the fundamental periods of the 20th century. The communal council decided to create the museum in 1998. It was inaugurated in 2005 with a positive response from the critics and the public. It holds over 800 examples of Italian and European toys of the 20th century.
The Touro College, thanks to an agreement with the Province of Rome and the City of Zagarolo, has started preparations to open a new branch at the Rospigliosi Palace. As an integral part of its project and a prelude to the opening of its new branch, Touro University, Rome, in collaboration with the City of Zagarolo, will be organizing, starting in Fall 2008, independent, not-for-credit English courses for the population of Zagarolo and the surrounding area.