The main body of the St Mary's Church was built in the late 12th century and was probably rebuilt in the 1240s, including a tower, porches and north aisle, after being damaged by Llewelyn the Great in 1220. An upper level of clerestory windows was added during the 15th century. The tower originally had a timber spire clad in lead, though it was removed in 1802 after becoming dangerous.
The church's interior has a fine timber panelled ceiling with moulded beams, rafters and ribs, and carves bosses at the intersections. It dates to c. 1500.
The church organ dates from 1737, by Harris & Byfield, with two keyboards.References:
The Basilica of Santa Maria in Trastevere is one of the oldest churches of Rome. The basic floor plan and wall structure of the church date back to the 340s, and much of the structure to 1140-43. The first sanctuary was built in 221 and 227 by Pope Callixtus I and later completed by Pope Julius I.
The inscription on the episcopal throne states that this is the first church in Rome dedicated to Mary, mother of Jesus, although some claim that privilege belongs to the Basilica di Santa Maria Maggiore. A Christian house-church was founded here about 220 by Pope Saint Callixtus I (217-222) on the site of the Taberna meritoria, a refuge for retired soldiers. The area was made available for Christian use by Emperor Alexander Severus when he settled a dispute between the Christians and tavern-keepers.
The church underwent two restorations in the fifth and eighth centuries and in 1140-43 it was re-erected on its old foundations under Pope Innocent II.