St Kyneburgha's is described by Pevsner as the most important Norman parish church in the county (i.e. Huntingdonshire). An aisleless cruciform church was built in the early 12thc. and dedicated in 1124. In the 1220s a S aisle was added and the chancel replaced; in the 1260s the S transept was replaced by a large chapel with an E aisle; and early in the 14thc. a N aisle was added. A broach spire was added to the tower around 1350, and the nave clerestoreys were inserted in the mid-15thc. The tower is of ashlar, the rest of the church of stone rubble.

Romanesque sculpture is found in the crossing arches and the exterior of the tower, in the W window of the nave, the reset S nave doorway, a tympanum reset over the S porch entrance, the dedication lunette set above the S priest's doorway and a pair of corbels set in the S porch. Part of a relief showing figures under arcading, now set in the N aisle, is discussed below but probably belongs to the 9thc.

The tower has two elaborately decorated storeys above a plain plinth storey, each of the three storeys being topped by a corbel table supporting a decorated frieze. The upper storey has five double units of blind arcading on each face, the three central units covering bell-openings. The area above the arches is diapered with fish-scale. The lower storey on each face has a central double arched window, flanked by a double unit of blind arcading to either side. Again the area above the arches is diapered, this time to give the effect of opus reticulatum. At each angle of each of the upper storeys is a nook-shaft.

The dedication to St Kyneburgha (Kyneburga, Cyneburga, Cyneburh) is unique in England. She was a daughter of King Penda of Mercia who founded the double monastery of Dormundescastre some time after the death of her husband Alhfrith around 664. A confirmation of the grant of lands to Peterborough (Medeshamstede) by Wulfhere, king of Mercia, in the same year includes Castor, but this is generally thought to be a post-Conquest forgery. In 1086 the manor was held by Peterborough Abbey, although remarkably there is no mention of a church.

The dedication lunette has usually been assumed to provide a date of 1124 for the completion of the church, but it does nothing of the kind for three reasons. First the date it gives is for the dedication of the church and nothing else. Second, by the mismatch between the careful relief carving of the characters as far as MC, and the sketchy inscribing of the remainder it appears to supply a rather earlier date (1100-1109) for its own production. The inscribed part of the dedication could have been added at any time after 1124, and was presumably not done until the slab was in place, otherwise greater care would have been taken with it. Finally, Rigold (1977, 103) following Ruprich-Robert argued that the date was not 1124 but 1114 on the basis of the epigraphy. The inscribed XX is in fact a single X with two cross-slashes (see plate). In this Rigold is followed by Fernie (2000).

The glory of Castor is the magnificent suite of figure and foliage capitals on the crossing arches, here dated c. 1100-1110. The capitals are simple blocks or, rarely, volute types. Figures are dynamic and stand out against their flat grounds. The forms of the dragons in particular (e.g on S tower arch, S face, 2nd order E capital) still retain elements of the Ringerike style prevalent in the reign of Cnut. The shields of the warriors (S tower arch, 1st order W capital) are of the Norman kite-shaped design familiar from the Bayeux tapestry. Special emphasis has been given to the W crossing arch, which is the only one with volute capitals, and the only one with sawtooth bases consistently used. The latter motif also occurs at Thorney (see Rigold (1977, 103), where the capitals are simple cushions and scallops.

The Castor workshop is readily identified at nearby Sutton and Maxey. Boase's comparison with the cloister sculpture of Reading Abbey is not particularly persuasive. A pair of warriors with shields and maces, similar to those on the S crossing arch, 1st order W capital is found on the font at Wansford. The style of the Christ in Majesty relief over the S porch is not incompatible with the figural capitals of the crossing, and may also be dated c.1100-1110.

The relief supposed to come from St Kyneburgha's shrine (IV.5.c), was discovered in 1924 in use as the riser of one of the altar steps in the church. Discussions of its date involve also the related reliefs in the chancel at Fletton, which have the same treatment of heads and drapery, especially around the hem, and similar feet. The presence of the earlier frieze at Fletton, which is indisputably linked to the Mercian work at Breedon on the Hill (Leics) has led most scholars to suppose that all of these figures are from the same early period. Clapham (1927), 235-56, dated them to the late 8thc., as part of a group of Mercian works centred on the Hedda stone in Peterborough Cathedral. His view has been more or less accepted by the majority of scholars, including Gardner (1951), 38. Kendrick (1938), 176-78 and Tweddle (in London, BM (1991), 239-40) preferred an early 9thc. date, and Stone (1955), 24, a date in the mid-9thc. Rice (1952), 88 was unable to decide between the late 8thc and the early 10thc, but had a slight preference for the former. RCHME preferred the later date, 'perhaps 10th-century' for the Fletton figures.

As early as 1927, however, the President of the Society of Antiquaries (David Lindsay, Earl of Crawford) expressed a preference for a 12thc. date for all of these reliefs (see Clapham (1927), 239-40). A curious position was taken by Pevsner (1968), who related the Fletton figures to 'French sculpture of the early 12thc.' while attributing the obviously related Castor stone to the mid-9thc. The treatment of hair is certainly classicizing, and the drapery patterns have the kind of schematised fold pattern for which a date in the 12thc would be perfectly acceptable. However, neither on these reliefs nor on the Castor one is there anything diagnostically Romanesque. The Castor figures stand under fictive arcading, and the capitals have an Anglo-Saxon bulbous form while the book-cover held by the main figure is decorated with a knotwork design. The present author leans toward the view that these are Anglo-Saxon, and from a period exposed to Carolingian classicism, i.e. probably the early 9thc.

Benefice of Castor with Sutton and Upton with Marholm.