Certosa di Pavia

This beautiful monastery from 14th century, near Pavia (50km south of Milano), is perfectly well conserved. There is a inside tour with one of the priests as guide, where you can admire its original architecture and rich decoration, and feel how was the certosini (hermits priests) life inside the monastery.
Certosa di Pavia official web site (only italian)
Comune di Pavia - Certosa (with english texts)