The present church of St Giles, Castle Street was rebuilt in 1875 to the design of T. H. and F.Healey of Bradford on a site slightly to the N of the medieval church that it replaced. The old church was founded 1092 (see History), and consisted of a nave, chancel, N transept and S porch, to which was added a large N annexe early in the 19thc. This was pulled down when the present church was built, but its chancel arch was re-used at the E end of the S nave aisle, and a doorway, perhaps based on the old S nave doorway but constructed of stones of various periods, was inserted at the E end of the N aisle to serve as the vestry doorway. These two features are described below.

The church of 1875 has a 4-bay aisled nave with clerestoreys. The aisles continue alongside the 2-bay chancel, providing a S chapel and a N vestry.

The history of St Giles begins with the foundation of Barnwell Priory of Augustinian Canons by Picot, Sheriff of Cambridgeshire at the time of Domesday and Lord of Bourn and Madingley, who built a church of St Giles by Cambridge Castle in 1092 in fulfilment of a vow made by his wife Hugoline when she fell ill. The house had 6 canons, but Picot and Hugoline died very shortly after the foundation and their son Robert fled overseas for his life after the rebellion of 1095, in which he was involved, failed. His holdings reverted to the crown, and the Barony of Bourn, to which the priory belonged, was eventually given by Henry I to Payn Peverel who increased its endowments and relocated the priory to Barnwell, a mile to the E. St Giles continued as a small church just outside the town walls.

In 1092 it lay within the diocese of Lincoln, but it passed to Ely when that diocese was formed in 1109.

The former chancel arch is dated ealy Norman by Bradley, and must date from the 1090s or a few years later. The Former doorway reset in the N aisle is much later work of c.1190-1210.