The environmental impact of the Late Iron Age and Romano-British ironworking hillfort of Bryn y Castell in upland southern Snowdonia was investigated by multiple profile pollen and charcoal analysis of nearby valley mire and blanket peat deposits. Pollen data, collected from five radiocarbon dated profiles within a 1.5 km radius of the hillfort, indicate that ironworking activities apparently had only localised impact on the environment. Small-scale declines in certain arboreal taxa can be correlated with occupancy of the site. Betula and Alnus appear to be most affected, with minor loss of Corylus and Quercus. The pattern of arboreal taxa and charcoal values during the ironworking period is considered in the context of evidence for deliberate woodland management, the scale and duration of ironworking, and alternative forms of human disturbance. Overall, the results of the pollen analysis suggest that woodland recovered to its pre-ironworking level except in the immediate vicinity of the hillfort. Integration of the archaeological and palaeoecological data allowed understanding of the resource-based aspects of this prehistoric industrial site and the results have wider implications for the field of experimental industrial archaeology.