The fountain was designed by Giovanni Fontana, whose brother had worked on the Fontana dell'Acqua Felice, and Flaminio Ponzio. They used white marble from the nearby ruins of the Roman Temple of Minerva in the Forum of Nerva, and constructed a massive gateway of five arches for the arrival of the water. At the top of the fountain are the papal tiara and keys, above the Borghese family coat of arms of an eagle and a dragon, supported by angels. The inscription praises Pope Paul in poetic terms for bringing water to the residents of the district.
Unlike the Fontana dell'Acqua Felice, which had an abundance of statues on biblical themes, the theme of the Fontana dell'Acqua Paola was water. The form of the fountain served as an inspiration for the later Trevi Fountain in Rome.
Read more: https://en.wikipedia.org/wiki/Fontana_dell%27Acqua_Paola