The monument includes the Church of St Marys situated within the walls od Portchester Castle (SU 60 SW 1). It was built for a priory of Augustinian canons founded in about 1128 by Henry I. The priory was removed to Southwick by 1150. The cloister lay to the south, between the church and the fort walls, but has been completely demolished. The only traces remaining are those of the lavatories of the reredorter which can be seen in the fort wall, and the scar where the west range joined the church. Built soon after 1130, the church is largely complete, and was little altered in its later phase as the parish church of Portchester. Early in the seventeenth century the south transept was removed and the chancel reconstructed by Sir Thomas Cornwallis. Fire damage to the building by Dutch prisoners in 1653 was finally made good by Queen Annes Bounty in 1706-10. The church is built to a simple cruciform plan, of ashlar blocks, with flat buttresses and plain round-headed windows. The building was restored in 1888.