The monastery (called St Seiriol's monastery) grew in size and had a wooden church building by the 10th century. This wooden building was, however, destroyed in 971 and then rebuilt in the 12th century in stone, from 1120 to 1123. The oldest parts of the Priory Church of St Seiriol date to 1140. It survived the initial Norman invasion of Gwynedd between 1081 and 1100, defended by Prince Gruffudd ap Cynan of Gwynedd. The priory church was enlarged in the early 13th century, at the time of the Augustinian Rule. There are records for the election of Priors in the "Calendar of Patent Rolls" back to 1306, when one Iowerth the Prior is mentioned. The dining hall was on the first floor, with a cellar below and dormitory above. In the 16th century, a kitchen and a warming house were added at the east of the building. The eastern range of buildings has gone, but the southern one, containing the refectory with a dormitory above, still stands.