In the late 10th or early 11th century a stone church was built on a simple plan consisting of a chancel and nave. The building includes much Roman material salvaged from the surrounding ruins of Verulamium, including Roman brick used in the splays of the nave windows. Early in the 12th century a north aisle and then a south aisle were added, linked with the nave by arcades of plain round-headed arches cut in the latter's north and south walls, leaving sections of Saxon wall as piers. The arcades do not match: the earlier north arcade has three bays spaced irregularly; the later south arcade was built with four bays. The round-headed Norman window at the east end of the north aisle may also be 12th-century.