The Byzantine castrum is situated under the ridge of the isle of St Marco, on the citadel itself and it dates back to the 6th century. A part of the castrum served as a Venetian watchtower until the 16th century. Sources mention two sacral edifices in the castrum (the churches of St John Chrysostomos and St Martin) whose remains, although hardly recognizable in the stone boulders, may be seen while driving a car across the Krk bridge. The castrum with an elongated layout obviously followed the configuration of the ground. It represented the so called portal of the island of Krk, a surveillance and tactical point of defence from attacks from the mainland. In old sources, the island appears under the name Almis, meaning fertile, which is completely opposite to its today’s condition with the prevailing naked stone.