Between 1580 and 1585, Mattei commissioned one of the most famous fountains in Rome; the Fontana delle Tartarughe (Turtle Fountain) in front of his residence.[1] In an original plan of 1570 a new fountain was to have been placed near the Theater of Marcellus in the Piazza Giudea, the site of a market in the Jewish ghetto. Mattei had been appointed to the committee which would select the site, though, and used his influence and money to have the fountain location changed to the small Piazza Mattei, in the block where the members of his family lived.[2][3] Mattei also offered to pay to have the piazza properly paved to ensure the sculpture would be well protected; "che il signor Mutio Matthei si obblighi a far mattonare" ("...that Mr Muzio Matthei undertakes to make paved").[4]
A popular Roman legend suggests Mattei, ruined by gambling, ordered the fountain to be built overnight in order to impress the wealthy father of a woman he wished to marry. The next morning he opened the window of his palazzo and showed his future father-in-law the fountain. The father, suitably impressed, allowed the marriage to go ahead, and Mattei, to remember the event, had the window overlooking the fountain closed up. A window closed by brick still overlooks the fountain.[5]