Bryn y Castell is a small hillfort near Ffeistiniog, Merionnydd, located on the Roman Road known as Sarn Helen. The fort is enclosed by a stone wall about 24 by 32 meters. Excavations revealed evidence for iron smelting and smithing, dating between 50BC and 50AD.

The fort’s defenses were always relatively simple, consisting of a stone rampart encircling the top of the hill. The original 2m-wide gate near the north-east corner was blocked early in the history of the occupation and a new one made 9m further west. The fort dominates this particular valley, but it is not generally visible from elsewhere.

The fort contains multiple roundhouses, including one that resembles a spiraling snail shell, which was adapted as a smithy from an earlier roundhouse. Based on these postholes, archaeologists believe that two of the round houses were wooden, which is deemed unusual, since it’s rare to find evidence of roundhouses built in anything but stone. The interior ground was once cobbled, with two iron furnaces found in the southern area. Another furnace was discovered immediately outside the north wall. This furnace was in use during the second to third century AD, after the furnaces in the hillfort itself were no longer being used for smithing. It is not clear whether the fort was still occupied at this time.

A total of about 1200 kg of slag was found at Bryn y Castell, significantly more than was known from any prehistoric site at the time of excavation. This produced only about 100 kg of finished bar iron, which also would have required 100 kg of charcoal. Archaeologists view this cost as evidence of the value of iron.

The ore to feed the furnaces was probably obtained from the iron deposits in the valley to the north-east. The fact that the Roman road, Sarn Helen, runs by the fort might explain why the fort was abandoned in 50 AD and then reinhabited after the local garrison left in the second century.