In 1153, Pope Anastasius IV gave the territory to the trappist monks of the Abbazia di Sant’ Anastasio alle Acque Salvie who built the castle in 1255, around which a residential area slowly grew.
For more than a century, the monks held the much contested control of the territory until 1378, when Genzano witnessed a turbulent history of ownerships and the consequent domination of numerous families: Orsini, Colonna (1393), Captain Pietro Passarelli, once again the Colonna, Massimi and Cesarini.
In the 17th century the town endured a splendid and flourishing period thanks to Giuliano III Cesarini: the layout of the Olmate, three long boulevards lined with elm trees, the restructuration of the baronial palace and the rebuilding of the Chiesa Santa Maria della Cima.
Four years after his death, in the absence of legitimate heirs and the elder daughters confined to convent, the brother of Giuliano III, clergyman Filippo Cesarini, inherited the property. A few years later Livia, the second daughter of Giuliano III, left the convent to marry Federico II Sforza, which led to the beginning of the Sforza – Cesarini dynasty.
Lady Livia commissioned many important architectural and urban works forming the New Genzano. A second trident was added to the already existing one, a “built” trident connecting the Via Livia (now Via Italo Belardi), the Via Sforza (now Via Bruno Buozzi) and the avenue that leads to the Convento dei Cappuccini (now Via Garibaldi). This new urban layout, based on a triangulation system, was planned by the important architects of that time , Tommaso Mattei and Ludovico Gregorini.
In 1778 (or as others claim in 1782), the infiorata (an allegorical carpet of flowers) took place for the first time on the initiative of two brothers Arcangelo and Nicola Leofreddi.
1780 was the year of the important opening of Via Corriera, also known as Via Postale that connects Genzano to Naples. During the years the development and population increase of the town led to its expansion over the surrounding planes, creating its actual configuration.
After the Restoration and end of Feudal rights, Genzano was purchased by the Holy See who named it the chief town over Nemi, Lanuvio and Ardea. On 23rd September 1828, Pope Gregorio XVI granted Genzano the title of city and after the capture of Rome it became part of the Italian State.
The town was destroyed by Second World War bombings causing the death of many victims