San Caio was a 17th century conventual and titular church which was demolished allegedly to make way for the Defence Ministry on the south side of what is now Via XX Settembre, at the junction with Via Firenze. This is in the rione Castro Pretorio.
The pope was allegedly from Salona, near Split in what is now Croatia, and some Croat noblemen came to Rome in the 17th century to investigate his background. They were especially interested in tracing the ancient titulus Gaii, a very early parish church dedicated to him, and believed that they had located it in ruins near the Porta Pia.It is thought, on the evidence available, that this site in ancient times was occupied by a building called the Domus Nummiorum in the first part of the 4th century. This had a Mithraeum in its basement. On the ruins of this a church was built perhaps in the 5th century, but this in turn fell into ruin in medieval times as the area was depopulated.
In 1631 Pope Urban VIII authorized the building of a church, the architects being Francesco Peparelli and Vincenzo della Greca. The shrine of the saint was established there, and the church became a Lenten station as well as being granted a cardinalate title. There was a convent of Cistercian monks attached.
By the time of the Nolli map of 1748, the Cistercian monastery had failed and the church was in the care of the convent of Incarnazione del Verbo Divino next door. The nuns must have restored at least the façade in the earlier part of this century.
In 1873 the complex was sequestered by the Italian government, and completely demolished in 1885 without archeological investigation. This was not in order to make way for the Ministry of Defence buildings, because a photo survives of 1883 showing the Ministry complete and the church still there. Rather, it was in order to make way for the Via Firenze.
The church originally stood back from the street frontage. It was exactly where the Via Firenze is now, and the façade line is marked by the edge of the pedestrian crossing further from the junction.
A watercolour by Achille Pinelli of 1834 survives (see the De Alvariis gallery in the "External links"), but needs to be treated with reserve as this artist is known to be unreliable in reproducing architectural details. The old photo on the Romasparita link looks as if it had been taken from the Via Barberini, looking up the Via San Nicolà di Tolentino.
The convent was tiny. There was a small block attached to the right hand side of the church, and a wing attached to the rear of the church beyond which was a small garden. To the right of the church was a courtyard (hardly a cloister), and beyond that was another detached wing.
The church had the plan of a Greek cross, with a nave and two large side chapels. Attached to the far arm of the cross was a large rectangular presbyterium or apse entered through a triumphal arch.
The entrance was up a short flight of stairs, hinting that there was a crypt. The façade was of two storeys. The first one had four pilasters with plinths and, unusually, imposts but no proper capitals. These supported an entablature, the central part of which was brought forward over the entrance. The latter had a pair of Doric pilasters without plinths, supporting an archivolt enclosing a tympanum which had a short inscription enclosed by two incurved curlicues meeting at the top. It read: Urbanus VIII Pont. Max.
At the top of the archivolt was a shield showing the Barberini bees, and on the lintel of the doorway was a flying eagle in relief. The shield was a reminder of the patrons of the Incarnazione convent, which had responsibility for the church for most of its existence.
The second storey was quite a complicated piece of design. The central zone was brought forward, and this focused on a large rectangular window with a Baroque frame. Over this was a raised segmental pediment, which was enclosed in and protruded from in the tympanum of the triangular pediment that crowned the storey. This main pediment had a central cross finial, and two flaming urn finials at each end.
The rest of the second storey was decorated with a set of blank framed panels. A very small horizontally rectangular one was under the window, and a pair of large and long vertical rectangular ones flanked the protruding central zone below the pediment. Below these two were two other small, horizontally rectangular panels which were brought forward to the same plane as the central window. A further two of the same design were to left and right of these, but they were not brought forward and had a pair of gigantic incurved volutes framing the upper part of the storey above them. See the image of the watercolour by Pinelli on the De Alvariis web-page to make sense of all of this.
The campanile was placed over the left hand side chapel. It had a tall single storey, with a large arched soundhole on each face except that facing away from the street. The corners had pilasters with thin imposts but no capitals, and the arches also had pilasters with prominent block imposts. There was a projecting cornice, and a hemispherical lead cupola.
The relics of St Caius were enshrined under the main altar, and the altarpiece of this showed St Caius Administering a Baptism by Giovanni Battista Speranza (1600-40). The left hand side chapel was dedicated to St Mary Magdalen, and the altarpiece was The Resurrected Christ Appearing to St Mary Magdalen by Mario Balassi. The right hand side chapel was dedicated to St Bernard, a reminder of the monks who were originally responsible for the church. It had an altarpiece by Andrea Camassei showing St Bernard's Vision of Christ with his Virgin Mother.