St Mary's Church
In the southeast corner of the castle there is an old Augustinian priory chapel, it is now known as "St Mary's, Portchester". The chapel was built in the twelfth century of ashlar blocks to a simple cruciform plan with flat buttresses and plain round-headed windows, and the original "Norman" west door can still be seen.
The cloister buildings were demolished and the south transept was removed in the seventeenth century. At one time the Chapel had a kitchen and refectory, a cloister or covered walkway, sleeping quarters and storage buildings. Some evidence still remains of the toilets or reredorter set in the southern side of the Roman wall and a fireplace near the Watergate.
The Augustinian Canons, who wore black robes and black caps, moved to the village of Southwick by 1155 due to lack of space, they built the priory which was one of just six Augustinian priories in England at that time.