The Ring of Iron (Welsh: Gylch Haearn) or Iron Ring of Castles was a chain of fortifications and castles built across Wales at Edward I's command after the death of Llywelyn ap Gruffudd in 1282 and the subsequent Conquest of Wales by Edward I of England. Edward spent over £80,000 on all of the castles, with £20,000 being incurred just by Rhuddlan Castle, Aberystwyth Castle, Flint Castle, and Builth Castle.
The purpose of building the iron ring of castles was to control the native Welsh population, particularly in Gwynedd, which had been the main focus of resistance to the English crown for centuries. The jewel in the crown of the Iron Ring was Caernarfon Castle, which Edward intended to be his seat in Gwynedd and where he arranged for his son Edward II to be born.