St Mary's was erected within the town walls to serve as a church for the garrison of Caernarfon Castle. It was built in 1307 by Henry de Elerton, second master mason of the castle. The tower is incorporated in the north-west corner of the town walls, and it is the vestry for the church. St Mary`s became a chapel to nearby Llanbelig Parish Church. There is a Jesse Window. Much restoration work took place in 1814 by the architect Benjamin Wyatt.
