Cronk Sumark is an Iron Age hillfort rising abruptly above Sulby Claddaghs near the entrance to Sulby Glen, with sheer natural defences to the north and west and two encircling banks protecting the more vulnerable south and east slopes. The hill has two summits (the western one truncated by later quarrying) and the inner rampart shows evidence of vitrification, suggesting it was subjected to intense heat at some point in its history.