San Nicola in Carcere
(St Nicholas in Prison) is a titular church in Rome near the Forum Boarium in rione Ripa. The church is dedicated to St Nicholas of Myra, the patron saint of sailors and of children and the remote cause of the phenomenon of Santa Claus. (The reference to ‘Prison’ is obscure.)
For reference, a plan of the church is available here
The first church on the site was probably built in the 6th century, and but the first definite reference to the church is from a plaque on the church dating to 1128 (on the fluted column to the right as one enters) recalling its rebuilding and consecration.
It was constructed in the ruins three pagan temples from the Republicna era (2nd century BC) dedicated to Janus, Juno Sospita, and Spes (see plan here
) and the ancient Forum Olitorium (a vegetable market), and you can see fragments from them reused in the church.
The dedication to St Nicholas was made by the Greek population in the area. 'In carcere' probably refers to a tradition, supported by Pliny's history of Rome, that there was once a prison here, built in the ruins of the temples.
The church was rebuilt in 1599, when the present Mannerist façade was added, and restored in the 19th century on the orders of Pope Pius IX.