On the islet of Sveti Marko, next to Krk Bridge, we can find the remains of a Byzantine castrum (fortress). At this point, the fort had a tactical position and kept an eye on potential attacks from the mainland as well as the coastal sea route. The Venetians used part of the castrum as an observatory until the 16th century. Some sources state that there were also churches dedicated to St. Martin and St. John Chrysostom. Since the island of Sv. Marko is a protected area, unfortunately the remains of the fort can no longer be visited.