Llantwit Major, United Kingdom

The site was again excavated between 1938 and 1948. It may have been first settled in the 1st century, but the first stone structure was not erected until a hundred years later. The site developed slowly and, it has been suggested, was even abandoned for a while during the 3rd century. By the 4th century, there was an L-shaped villa with a large, aisled building possibly for farm workers, and a number of smaller agricultural structures almost enclosing a central courtyard. The Royal Commission on the Ancient and Historical Monuments of Wales has associated collection records of the site, including drawings of other remains such as statues and a tessellated pavement, as well as documentation of a 1971 excavation. An early-medieval-period cemetery is in evidence, as are earthworks, traces of walling, a bank and a ditch. Pieces of pottery have been found.

According to the 18th-century historian of ill-repute, Iolo Morganwg, the Côr Tewdws or 'College of Theodosius' was established in the late 4th century at a place at or near Llantwit Major called Caer Worgorn by co-founders Emperor Theodosius and Custennin Fendigaid, but was later burnt to the ground by Irish pirates and abandoned. Morganwg's works have since been discredited.

Germanus of Auxerre and Lupus of Troyes made at least one visit to Britain in AD 429 and founded or refounded a number of ecclesiastic establishments across South Wales. It was at this time that Saint Illtud came to the Hodnant valley. Although he is said to have been born in Brittany, Illtud had fought as a soldier in Wales before renouncing his former life at the behest of Saint Cadoc. What is certain is that Illtud oversaw the growth of a pioneering monastery and associated college called Bangor Illtyd, on the Ogney Brook, close to the current St Illtyd's Church. Together these institutions became the first great hub of Celtic Christianity and teaching.

Under Illtud and his successors, Bangor Illtyd and the monastery grew in reputation both as the origin of many prominent Celtic evangelists and as a major centre of scholarly education. As such, it began to attract scholars from across the Celtic and wider world. The college itself was said to be the oldest in the world, consisting of seven halls, 400 houses and more than 2,000 students at its peak, including seven sons of British princes, and scholars such as St. Patrick, St. Paul Aurelian, the bard Taliesin, Gildas the historian, Samson of Dol, and St. David are believed to have spent some time there. Samson was known to have been summoned by Dyfrig to join the monastery in 521 and he was briefly elected abbot before leaving for Cornwall. King Hywel ap Rhys (d. 886) was buried at the monastery.

The college suffered during the invasions of the Saxons and the Danes, and was destroyed by the Vikings in 987 and again by the Normans in the late 11th century. In 1111 it is documented as being restored, but likely in a lesser state than the original. It is known to have continued to function as a monastic school until the 16th-century Reformation. The ruins of the original school house are located in a garden on the northern end of the churchyard and the monastic halls were located in a place called Hill-head on the north side of the tithe-barn. Although nothing of the original monastery remains, the present church was originally built between 950 and 1400 and its earliest existing secular buildings date from the 15th century. The church and school became the property of Tewkesbury Abbey around 1130 after becoming part of the Norman kingdom of Glamorgan.

After the dissolution of the monasteries by Henry VIII during the Reformation, it became independent from Tewkesbury in 1539. St Donat's Castle, 1+1⁄2 miles (2.4 km) to the west, was built in the 13th century.

In the 20th century, Llantwit developed into a dormitory town and grew about 15 times in size to accommodate the Royal Air Force at St Athan. Despite its modernization and rapid growth, it retains its pre-modern feel with its narrow winding streets, high walls, old town hall and gatehouse, and several inns and houses dated to the 16th century. Llantwit Major railway station on the Vale of Glamorgan Line was reopened in June 2005. In 2014, it was rated one of the most attractive postcode areas to live in Wales.