The palace is the evolution of the Renaissance fortress of Castel Gandolfo and the works were initiated by Pope Urban VIII Barberini and completed in 1626. Following that, the gardens and the square were placed in front of the main entrance. Over the centuries the building has been renovated and enlarged several times. Alexander VII modified the main facade and the west wing of the building. Benedict XIV decorated the main gallery with views of the Alban Hills. Pius XI rebuilt it almost completely, leaving only the facade intact. Inside several rooms are decorated with precious frescoes of the Zuccari. Since 1936, it has been home to the Vatican Observatory, the observatory and research center, founded by Gregory XIII in 1585. The gardens extend over the remains of the villa of Emperor Domitian, and in one of the outputs of the palace is a marble equestrian statue representing the same Domitian. The pontiffs are accustomed to travel to Castel Gandolfo at least once a year.