The so-called Temple of Diana was part of the Roman sacred complex but it was not a temple, rather it was a library that originally faced onto a portico that enclosed much of the spring sanctuary. The date is uncertain; some scholars suggest the first century, others the second. It was used as a church from the Middle Ages till the 16th century when it was damaged in the Wars of Religion.
References:The Villa d'Este is a 16th-century villa in Tivoli, near Rome, famous for its terraced hillside Italian Renaissance garden and especially for its profusion of fountains: the extraordinary system contains fifty-one fountains and nymphaeums, 398 spouts, 364 water jets, 64 waterfalls, and 220 basins, fed by 875 meters of canals, channels and cascades, and all working entirely by the force of gravity, without pumps. It is now an Italian state museum, and is listed as a UNESCO world heritage site.
Tivoli had been a popular summer residence since ancient Roman times due to its altitude, cooler temperatures and its proximity to the Villa Hadriana, the summer residence of the Emperor Hadrian I.
The Villa was commissioned by Cardinal Ippolito II d'Este (1509-1572), second son of Alfonso I d'Este, the Duke of Ferrara and grandson of Pope Alexander VI, along with Lucrezia Borgia.