Jonathan's journey takes him to Caernarfon Castle, built in 1283 and one of the most magnificent examples of medieval castle building in Britain. Jonathan, aided by top British climber Lucy Creamer, tests his climbing skills to reveal the secrets of this monster of a castle's construction, and what it would have been like for anyone foolhardy enough to attack it. On his climbs, he scales over 100 feet up the majestic Eagle Tower to investigate how the architect behind Caernarfon revolutionised castle building in Britain; he discovers how the castle's design and decoration were inspired by the ancient Roman town of Constantinople; and he tests the limits of his courage zip-wiring high above the castle before abseiling down the King's Gate to investigate the fortified main entrance, which housed an innovation so lethal that it's been dubbed a medieval machine-gun.