St Mary's Church
In the southeast corner of the castle there is a 12th Century Augustinian priory chapel, now known as St Mary’s, Portchester. It was built 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 have been demolished and the south transept was removed in the 17th century, but at one time it had a kitchen and refectory, a cloister, sleeping quarters and storage buildings. Evidence including the toilets set in the southern side of the Roman wall and a fireplace near the Watergate still remain.
The priory was built by the Augustinian canons (who wore black robes and black caps). It was one of just six Augustinian priories in England at this time. By 1155, the Augustinian canons had moved to the village of Southwick due to lack of space.