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He aided Overbeck in carrying out the frescoes of the Evangelists and Apostles at Castel Gandolfo, and at a later date, when Overbeck's strength was no longer equal to the task, Seitz, with the aid of his son, Ludwig Seitz, completed Overbeck's frescoes in the cathedral at Diakovar by filling the gaps with compositions of his own.
He completed the stucco decoration of San Tomaso di Villanova in Castel Gandolfo (1660-1), the stucco decoration of Bernini's Sant'Andrea 1662-1665), the statues of Saint Bernardino and Pope Alexander VII Chigi for Duomo di Siena and the Virgin and Child in Saint Joseph des Carmes in Paris (1650–51).
According to the Lateran Treaty, certain properties of the Holy See that are located in Italian territory, most notably Castel Gandolfo and the major basilicas, enjoy extraterritorial status similar to that of foreign embassies.
The orchestra under Maestro Barenboim, in the presence of President and Mrs. Giorgio Napolitano, performed for Pope Benedict XVI at the courtyard of the Apostolic Palace of Castel Gandolfo on Wednesday, July 11, 2012, the abbot Saint Benedict of Nursia's (the founder of the Benedictines) feast day, and thus the name day of the Pope.