An impressive network of underground passageways and blockhouses stretches under the foundations of St John's cathedral, of which only the ruins now remain. Learn how this network helped detect Turkish attempts to blow up the castle through the digging of tunnels to smuggle in explosives. Chickpeas placed on a taut drumskin, which reacted to even the slightest air disturbance created by the assailants, served as a warning to the besieged forces.