He built one of the most important rural villas of the Roman Campagna in 1601-1605 for the Aldobrandini family. Castello di Torrenova was originally a medieval farmhouse that Fontana enlarged and embellished with Renaissance details and crenellated walls. Next to the castle a small late Renaissance church was built dedicated to Saint Clement, the patron saint of the Aldobrandini Pope, Clement VIII.
Fontana is the first great architect-engineer who carves out a role in hydraulics that deals with the leveling of land, the conduct of canals, and the control of the speed of water. He brought water back to Rome by reorganizing and exploiting the hydraulic works of the past.[2] Around 1612, Pope Paul V commissioned Fontana to design the Fontana dell'Acqua Paola on the Janiculum to create a source of clean drinking water for the residents.[6] The form of the fountain served as an inspiration for the later Trevi Fountain.