The present church, built in 1839 was constructed in a North – South direction whilst the Old church was in an East – West direction with a nave that dated back to the Saxon era. It was the oldest part of any church still standing in Lewes and built about the time of King Alfred. On the outside wall of the old nave was a unique plaque in Latin dedicated to Prince Magnus of the Royal House of Denmark who became an anchorite here in Lewes. This medieval Lombardic inscription was later incorporated in the East Wall of the New Church. The rest of the old church was built on the Saxon foundations in medieval times. The area of the churchyard formed an entrenched angle in the town’s medieval defences and is believed to be part of the Earthworks of a Roman fort built to guard the old river crossing.

The 1851 Religious Census of St John Sub Castro:

Number attending church - Evening 800, This is given as an average number

John Cooke, Registrar, Lewes - 9th September 1851