San Nicola dei Cesarini is a lost Baroque parish and conventual church that used to stand in what is now the cat-decorated display of ancient ruins in the middle of the Largo di Torre Argentina in the rione Pigna. Some architectural fragments survive. Picture of the church at Wikimedia Commons. 
The dedication was to St Nicholas of Myra.
It is thought that the church was founded in the 9th century, since the lowest level of its excavation revealed a semicircular crypt and some brick piers which seem to date from the period. The plan was of a nave and two aisles. An ancient temple from the Republican period, dating from the middle of the 3rd century BC and thought to have been dedicated to Juturna, was utilized. Portions of the temple’s peripteral colonnade were incorporated into its outside walls.
However the first documentary attestation is of the 12th century, when the church was rebuilt. It is listed as dependent on the parish church of San Lorenzo in Damaso in 1186. The main apse, which has a smaller apse adjacent for a side chapel, survives from this rebuilding and has been preserved. Some Cosmatesque pavement was also excavated after the 20th century demolition.
Back then, the church’s name was de Calcario, or “of the lime kiln”. This was a reference to a local mediaeval industry, which involved burning useless bits of marble (such as classical statuary) from ancient ruins in order to make lime for plaster, cement and fertilizer.
The church was rebuilt in 1611, but badly and it had to be rebuilt in a Baroque style in 1695. The name was changed to dei Cersarini in honour of a noble family whose palazzo was adjacent. The latter rebuilding was when the church became the Roman headquarters of the Clerks Regular of Somasca, who established a small convent to the south. They moved here because their former convent at San Biagio a Montecitorio had been purchased by the Papal government, and demolished to make way for Curial offices.
However, it was known that church stood on the site of an ancient temple, and remains of a second one (the circular structure now visible) were apparent in the fabric of the convent. Hence it was doomed by the nationalist archaeological interest in these, and they was demolished in 1929 except for the surviving mediaeval fragments.
The Clerks Regular are now based at Sant'Alessio all'Aventino.
The plan was rectangular, with an additional shallow rectangular apse entered through a triumphal arch. There were two side altars in niches in each of the side walls.
The church was oriented west to east, and fronted onto a fairly large square piazza. A passage ran from this around the left hand side of the church to the Palazzo Cesarini, which was immediately behind the apse. The convent was to the south of this passage, and was rather cramped. The buildings occupied three sides of an odd-shaped courtyard; it was mostly rectangular, but the south-west corner was an arc forming a quarter-circle. This was on the footprint of the ancient circular temple, and three columns were embedded in the wall. The south side of this court belonged to another property.
The church façade had one storey, with two pairs of tall Composite pilasters running from ground level to the entablature. The latter supported a triangular pediment containing a fresco of the Madonna and Child. The doorcase consisted of two Ionic columns supporting an architrave and broken segmental pediment, and above the architrave was a coat-of-arms in a tondo. This had its own frame, of two little columns supporting a triangular pediment which was inserted into the doorcase pediment and intruded on the central round-headed window. The latter was crowned with swags and a winged putto’s head which touched the main entablature.
There were two round-headed niches in between the pairs of pilasters, with frames in the same style as the tondo, and these contained frescoes of St Nicholas and St Blaise. The latter was a reminder of the old church of the Clerks Regular, and the church had a subsidiary dedication to him.
The main altarpiece showing The Vision of St Nicholas was by Marco Benefial, and to either side of the apse were SS Peter and Paul by Paolo del Garofalo.
The right hand side chapels were dedicated to the Crucifixion and to St Blaise, and the altarpiece in the latter was by Avanzino Nucci. The near left hand side chapel (dedicated to Our Lady?) had an altarpiece by François de Troy, and the far one was dedicated to St Charles Borromeo with an altarpiece also by Nucci.
Surviving remainsEditThe surviving main apse has a mediaeval fresco fragment of the lower halves of three saints, which is suffering from being exposed to the weather. Here also is a 12th century altar, a cube in marble with pillars carved on its corners and a cavity in the top where relics would have been kept. The fresco fragment in the smaller side apse is in imitation of polychrome marble panelling in Classical style.
At present, access to the ruins seems to be best obtained by visiting the cat sanctuary established there (see external link below). It opens in the afternoons, and there are guided tours. Although there are fewer stray and feral cats than there used to be, owing to a sterilization programme (and, according to some locals, because certain immigrants eat them), this is not going to be an experience for ailurophobes.