It is a roman sepulchre cell of the Republican era, probably the tomb of Tulliola, daughter of Cicero. It had been adapted as a Christian chapel since the V century and presented to st. Nilus by the count of Tuscolo.

The cell windows still preserve the original Roman iron bars, whence the building takes its name (‘Crypta’ = cell; ‘ferrata’ = ironmade).

The inside garden leads to the Cryptoporticus, a magnificent portico in “opus reticulatum” which was part of the antique Roman Villa on whose site the Abbey was built.