Like nearly all of the oldest and largest parish churches in England, Christchurch Priory was originally part of an ancient Catholic institution, in this case an Augustinian priory, founded in 1150 by Baldwin de Redvers, 1st Earl of Devon, though the building was already in existence, construction having started in 1094, as a secular church. The history of the site is even older, however, back to the eighth century when a small Saxon priory was established, associated with the adjacent village of Twynham, which was renamed Christchurch soon after commencement of the new church, at the end of the 11th Century. Twynham means 'place between the rivers', and it occupied a naturally protected strip of land at the confluence of the rivers Stour and Avon,; the town also contains one other early Norman building, Christchurch Castle, which was established a decade or two before the church, initially as a wooden fort, later replaced with a stone keep.
Building work on the church occurred in stages, the last major components being the west tower, complete about 1480, by which time the priory was part of a traditional monastic complex also including cloisters and various subsidiary buildings, on the south side. All these were quickly demolished after the 1539 Dissolution of the Monasteries, but the church was spared, and is very little changed since; it is undoubtedly one of the finest parish churches from the medieval period, with beautiful architecture ranging in style from the Early English to the Perpendicular Gothic.
The early Saxon priory at Twynham was centred on a small, primitive church, which was removed following closure of the priory soon after the Norman conquest, to be replaced by the much larger building, work that started in 1094 under Ranulf Flambard (died 1128), a minister of William II. A legend tells that the intended position of the new church was 2 miles north on top of St Catherine's Hill, but the masonry was inexplicably moved one night to the river confluence site, as if by divine intervention. By the time of the conversion to the priory, the core of the church was already complete, with a nave, crossing, transepts, choir, apse, and (probably) a central tower. Over the next hundred years the nave aisles were completed, together with a north porch, a chapel off the north transept, and the clerestory above the nave and choir. In the 14th century the nave roof was raised and the east end of the choir extended by addition of the lady chapel, beneath an unusual vaulted roof with pendant shafts. Finally, in the 15th century, the central tower was removed, either by accident or design, replaced by a new tower at the west end, and the choir was rebuilt, merging it with the lady chapel. Several chantry chapels were also added around this time. Most of the church is built from pale-coloured limestone, including from Beer in Devon, Quarr in the Isle of Wight and Portland in Dorset.
The priory is mostly surrounded by gardens and the graveyard, and may be approached either from the north, from the town centre, or the southwest, where the nearest car park is located, at the end of Quay Street; just beyond is a marina along the River Stour. There is no trace of the old monastic buildings to the south apsrt from foundations of an outer wall, and part of the land here is occupied by a modern dwelling, Priory House. The church is 311 feet (95 metres) in length, the longest in England, and is best viewed from the north side. At the east end are the large windows of the lady chapel, and above are the smaller openings of St Michael's Loft, a rare example of an upper chamber. The exterior walls have generally plain masonry, with the exception of the north transept, which is embellished by a full-length arcade of intersecting blind arches at ground level, while the stair turret at the northeast corner has another three rows of graceful stonework, in contrasting styles. At the northwest corner of the transept are several slender columns rising to only two thirds of the height of the wall, presumedly broken off during some ancient modification. Entry to the church is through the north porch, which is unusually large, both in length and height; it incorporates a second story, and its roof rises well above that of the north nave aisle. The main doorway (13th century) is lined by marrow shafts of Purbeck marble, and its double doors are set beneath a deeply recessed, multi-tier arch.
The porch leads to the west end of the nave, which has eight bays, delineated by rectangular piers lined by narrow, half cylindrical columns. The westernmost pair are partly occupied by the sides of the 15th century tower, and are walled off to form small rooms. Above the columns and their connecting, round-headed arches is the trifolium, or gallery, framed by pairs of smaller arches, while the clerestory forms the upper level, illuminated by two-light windows. The nave roof, plaster with widely-spaced ribs, is from the 19th century. The two nave aisles are also lit by two-light windows, containing recent stained-glass; the south aisle is more elaborate, its lower wall decorated with an arcade of blind arches. The nave leads to the crossing between the two transepts, the junctions of which are supported by huge arches above massive columns, seemingly designed to bear the weight of a central tower, the former existence of which is unproven though presumed. The crossing is separated from the choir by a stone pulpitum probably from the 14th century, though extensively repaired in the 1840s. The transepts contain a variety of medieval features, and both connect to side chapels through their east walls.
The choir was completed in the late 15th century, in the Perpendicular Gothic style. It is lit by eight clerestory windows, four on each side, and its east end is dominated by the tall stone reredos (mid 14th century, designed for an earlier arrangement of the choir), consisting of three rows of canopied niches, all originally containing statuary, though most is missing. Above here is a pastel-coloured mural, painted in 1967. The choir roof is notable for its vaulting, since the lower end of the ribs extend downwards, away from the wall, forming decorated pendants, thought to be the earliest use of this style on the country. The easternmost of four bays along the north wall of the choir contains the chantry chapel of Margaret, Countess of Salisbury, who was executed in 1541 - a very ornate structure with a mix of Italianite and Gothic styling, including octagonal pilasters and an intricate, fan-vaulted ceiling. Three other, only slightly less ornate chantry chapels are for Sir William Berkeley (1486), along the north aisle, John Draper (1529), the last prior, at the east end of the south aisle, and Robert Harys, vicar of Christchurch (1525), in one of the bays of the south wall of the choir. The east end of the north aisle contains the marble tomb and alabaster effigies of Sir John Chidiock (died 1450), Sheriff of Dorset, and wife Katherine. The easternmost section of the church consists of the ambulatory and then the lady chapel, built in the Perpendicular Gothic style. This chapel has its own medieval reredos (also now lacking stautues) and altarstone of Purbeck marble; it is lit by three large windows. The roof is supported by the same perpendicular vaulting as used in the choir. In wall recesses either side of the altar of this chapel are two empty canopy tombs, traditionally attributed to Sir Thomas West (died 1405) and his mother Alice (died 1395), though the stonework has been dated only to the early 16th century.