Not far from St. Fortunato Church, in Parco della Rocca, San Cassiano prison, a quadrangular shaped building is placed. Basically, it is supposed to be an Ancient Roman cistern where, according to legend, the Bishop of Todi would have been imprisoned during the 2nd century A.D. persecutions. Later on, the room would have welcomed the remains of the Patron Saint, St. Fortunato and then, it would have been used as a Christian oratory.
Currently, the structure is an open space room, accessible from a small wooden entrance door, decorated with a rounded arch which is supported by two capitals, roughly made.
The side windows, as well, were added later on.