In Grottaferrata, a busy village a couple of miles from Frascati, the main attraction is a walled citadel founded by St. Nilo, who brought his group of Basilian monks here in 1004, when he was 90. The order is unique in that it's Roman Catholic but observes Greek Orthodox rites.
The fortified abbey, considered a masterpiece of martial architecture, was restructured in the 15th century by Antonio da Sangallo for the future Pope Julius II. The abbey church, inside the second courtyard, is a jewel of Oriental opulence, with glittering Byzantine mosaics and a revered icon set into a marble tabernacle designed by Bernini. The Farnese chapel, leading from the right nave, contains a series of frescoes by Domenichino.
If you make arrangements in advance you can visit the library, which is one of the oldest in Italy. The abbey also has a famous laboratory for the restoration of antique books and manuscripts, where Leonardo's Codex Atlanticus was restored in 1962 and more than a thousand precious volumes were saved after the disastrous Florence flood in 1966.