The core of the church is the original circular nave, now called the Round, with an annular aisle and an arcade supported on eight columns with early Romanesque capitals of various designs. The aisle wall retains one original respond with its capital. The columns now support chamfered pointed arches carrying an octagonal clerestorey wall pierced by square-headed double lights. This upper section belongs to a rebuilding of c.1375. To the E of the Round and reached by steps was originally the Romanesque unaisled chancel, terminating in an apse, and this remains as the nave and chancel of the present church. A two-bay N aisle was added to it c.1200. A second N aisle was added later, and in the 14thc. a S aisle was added. As it stands, therefore, the church has four parallel naves, terminating at their E ends with (from N to S) a vestry, the Chapel of St Thomas, the chancel with an apse, and the Chapel of St George, but it will be seen that much of the fabric is 19thc. At the W end, the original W doorway was demolished and a tower with a spire added in the 14thc. By the 17thc. only the Round was in regular use, and the rest of the church fell into disrepair. The choir and the outer N aisle were demolished. In 1851 the tower was struck by lightning, and in that year George Gilbert Scott was engaged to carry out a thorough restoration of the entire church. He rebuilt what had been lost, including the outer N aisle and the chancel with its flanking chapels, and the church was reopened in 1864. The vestry at the end of the N outer aisle was added in 1887. The Romanesque sculpture falls into two groups. The main arcade piers and their capitals belong to the original fabric of c.1110, as does the single remaining aisle wall vault respond. To these must be attached a small tympanum now set inside the aisle wall, and the corbels of the original chancel (now visible high on the inner walls of the S and inner N aisles). The N doorway of the Round is of c.1170-80.

The church was founded by Simon de Senlis, first Earl of Northampton, on his return from the first Crusade in 1099. It must have been complete by 1115 when it was granted by Earl Simon to the Cluniac priory of St Andrew's Northampton. Chaplains were provided by St Andrew's until 1226, when the Bishop of Lincoln imposed a permanent vicarage with a fixed stipend. The advowson remained with the priory until the Dissolution, except for the period from 1354 to 1380 when it was taken into the king's hands, owing to the war with France (St Andrew's being dependent on a French house).

Round churches are uncommon in England. Those established by the Knights Templars and Knights Hospitallers commemorate the rotunda of the Holy Sepulchre in Jerusalem, and so does this one although it was always a parish church and may be seen as a personal act of commemoration by Earl Simon. Pevsner lists those round churches known to have existed in this country. Of the Templars' foundations, the Temple Church in London survives, and others existed on the first Templar site in Holborn and in Bristol, Dover, Asleckby and Temple Bruerne in Lincolnshire, and Garway in Herefordshire. Of the Hospitallers' foundations, parts of St John's, Clerkenwell and St Giles, Hereford remain, along with the 14thc. church at Little Maplestead, Essex. Other round churches with no connection to the Knights are Holy Sepulchre, Cambridge, Ludlow Castle chapel and the excavated remains found at W Thurrock, Essex. Of all these, the Cambridge church is closest in date and appearance. Keyser contents himself by describing the tympanum as "a human figure between a smaller figure and a dragon," and in view of the confusion of limbs and the ambiguity of the attributes there seems little more to be said. For Leleux, however, it shows "a human figure holding a short sword in each hand, while on one side a dragon holds his arm and on the other a humanoid figure holds a torch," and he provides a sketch interpreting the carving along these lines. Leleux's interpretation is of "the forces of good and evil contending for the soul of a man," and he relates the dragon to Psalm 44, 18-19, "our heart is not turned back neither have our steps declined from thy way; though thou hast sore broken us in the place of dragons, and covered us with the shadow of death." The corbels have too many correspondences with those at St Peter's, Northampton not to be by the same workshop. Despite their crude style the imagery is inventive. Attention is drawn to the gryllus, N11, and the two human figures gripping objects in their arms.