Monastery at Llantwit Major

Cor Tewdws or Bangor Tewdws (Meaning "college" or "chief university" of Theodosius) was a Celtic monastery and college in what is now Llantwit Major, Glamorgan, Wales. It is believed to have been founded c. 395, making it the oldest established school in Great Britain. Today, the ruins of the monastery and college are believed to be under the St. Illtyd's Church in Llantwit Major.

The original college was said to have been established by (or in honour of) the Roman Emperor Theodosius I, and was named after him. In 380, Theodosius declared Nicene Christianity the only legitimate Imperial religion, ending support for traditional polytheistic religions and customs. The College of Theodosius is believed to have been founded c. 395, making it the earliest school, former or extant, in all of Great Britain. It has even been called "the oldest college in the world". It was founded during the last years of the Roman occupation of Britain, which ended c. 409 - 410 AD. After the withdrawal of the Roman legions, Britain suffered incursions of the Irish, Scots, and Picts, who raided British settlements, sacking villages and carrying off the inhabitants as slaves. The college was reputedly burnt down in 446, with different sources attributing the destruction to Irish pirates and Saxon war bands. Though there is a lack of contemporary sources, later writers state that St Patrick was kidnapped by Irish pirates while at the College.

After the sack of the College of Theodosius, the site lay barren for 62 years, until it was re-established by St. Illtud c. 508. According to the Book of Llandaff, St. Dubricius commissioned Illtud to re-establish the college, and the place came to be known as Llanilltud Fawr.

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