Merthyr Tydfil, United Kingdom

The Silures resisted this invasion fiercely from their mountain strongholds, but the Roman army eventually prevailed. In time, relative peace was established, and the Penydarren fortress was abandoned by about 120 CE. This was bad for the local economy, which had come to rely upon supplying the fortress with beef and grain, and imported items such as oysters from the coast. The Romans had intermarried with local women and many auxiliary veterans had settled on farms locally.

With the decline of the Roman Empire, Roman legions were withdrawn about 380 CE. By 402, the Roman army in Britain consisted mostly of Germanic troops and local recruits; the cream of the army had been withdrawn to the continent of Europe. Sometime in that period, Irish Dalriadans (Scots) and Picts attacked and breached Hadrian's Wall. During the 4th and 5th centuries the coasts of Cambria (Wales) had been subject to the raids of Irish pirates, in much the same way as the south and east coasts of Britain had been raided by Saxon pirates from across the North Sea. Around the middle of the 5th century, Irish settlements had been established around Swansea, the Gower Peninsula, Carmarthenshire, and in Pembrokeshire and eventually petty kingdoms were established as far inland as Brecon.

The Latin language and some Roman customs and culture became established before the withdrawal of the Roman army. The Christian religion was introduced throughout much of Wales by the Romans, but locally it may have been introduced later by monks from Ireland and France, who made their way into the region following rivers and valleys.

Local tradition holds that, around 480 CE, a girl called Tydfil, daughter of a local chieftain named Brychan, was an early local convert to Christianity, and was murdered by either Welsh or Saxon pagans, and buried in the town. The girl was considered a martyr after her death. Merthyr translates to "martyr" in English, and tradition holds that when the town was founded, the name was chosen in her honour. A church was eventually built on the traditional site of her burial.

For several hundred years the valley of the River Taff was heavily wooded, with a few scattered farms on the mountain slopes. Norman barons moved in after the Norman Conquest of England, but by 1093 they occupied only the lowlands; the uplands remained in the hands of the local Welsh rulers. There were conflicts between the barons and the families descended from the Welsh princes, and control of the land passed to and fro in the Welsh Marches. During this time Morlais Castle was built two miles north of the town.

No permanent settlement was formed until well into the Middle Ages. People continued to be self-sufficient, living by farming and later by trading. Merthyr was little more than a village. An ironworks existed in the parish in the Elizabethan period, but it did not survive beyond the early 1640s at the latest. In 1754, it was recorded that the valley was almost entirely populated by shepherds. Farm produce was traded at a number of markets and fairs, notably the Waun Fair above Dowlais.

Merthyr was close to reserves of iron ore, coal, limestone, timber and water, making it an ideal site for ironworks. Small-scale iron working and coal mining had been carried out at some places in South Wales since the Tudor period, but in the wake of the Industrial Revolution the demand for iron led to the rapid expansion of Merthyr's iron operations. By the peak of the revolution, the districts of Merthyr housed four of the greatest ironworks in the world: Dowlais Ironworks, Plymouth Ironworks, Cyfarthfa Ironworks and Penydarren. The companies were mainly owned by two dynasties, the Guest and Crawshay families.

Starting in the late 1740s, land within the Merthyr district was gradually being leased for the smelting of iron to meet the growing demand, with the expansion of smaller furnaces dotted around South Wales. By 1759, with the management of John Guest, the Dowlais Ironworks was founded. This would later become the Dowlais Iron Company and also the first major works in the area. Following the success at Dowlais, Guest took a lease from the Earl of Plymouth which he used to build the Plymouth Ironworks. However, this was less of a success until the arrival in 1763 of a "Cumberland ironmaster, Anthony Bacon, who leased an area of eight miles by five for £100 a year on which he started the Cyfarthfa Ironworks and also bought the Plymouth Works". After the death of Anthony Bacon in 1786, the ownership of the works passed to Bacon's sons, and was divided between Richard Hill, their manager and Richard Crawshay. Hill now owned the Plymouth Iron Works and Crawshay the works at Cyfarthfa. The fourth ironworks was Penydarren, built by Francis Homfray and his son Samuel Homfray in 1784.