Castles are arguably the British Isles' strongest tourist magnets -- in Scotland, England, Wales and across the Irish Sea in Ireland as well.

There, on the banks fo the River Shannon, King John's Castle overlooks Limerick city.

"There are many castles in Ireland, but King John's Castle is a castle with a difference because it's a military-style castle and 'twas built for military purposes by the Normans in the year 1200 and completed in 1212," says guide Patsy Reale.

The Normans chose their site well -- an archaeological dig has revealed evidence of a much older Viking settlement beneath the castle walls.

King John's Castle sits within a loop of the River Shannon called the King's Island, built to defend the westernmost portions of Anglo-Norman King John's empire. Its high castle walls and double-towered gatehouse were designed to defend against the 13th century's siege technologies.

An hour or two is easily spent walking through the courtyard, climbing the turrets and lookout towers -- which at times feels like walking in a scene from 800 years ago.