The post road built by Thomas Telford from London strengthened Holyhead's position as the port from which the Royal Mail was dispatched to and from Dublin on the Mail coach. The A5 terminates at Admiralty Arch (1822–24), which was designed by Thomas Harrison to commemorate a visit by King George IV in 1821 "en route" to Ireland and marks the zenith of Irish Mail coach operations. Holy Island and Anglesey are separated by the Cymyran Strait which used to be crossed on the Four Mile Bridge; so called, because the bridge was from Holyhead on the old turnpike.