Oratorio - PiP please

Oratories seem to have found their origin in chapels built at the shrines of martyrs, for the faithful to assemble and pray on the spot. The oldest extant oratory is the Archiepiscopal Chapel in Ravenna (c. 500). The term is often used of very small structures surviving from the first millennium, especially in areas where the monasticism of Celtic Christianity was dominant; in these cases it may represent an archaeological guess as to function, in the absence of better evidence.
And because they still believe the feet of the saints shall not touch the Earth . . . PiP . . .