Just outside the village of Kosoikia Evdilou in the northern part of the Ikaria, lie the remains of the Castle of Koskina, at the peak of Mount Koskinas. Also known as the castle of “Messarias” or “Nikarias”, the structure was once a strong fortress during the Byzantine Era and later, the Frankish rule. The Byzantines built the castle at the top of a conical hill around the 10th century A.D. Due to the excellent visibility it offered, a small military base was established at the site, allowing soldiers the ability to survey the entire area, both over land and sea. Today, multiple fragments survive. The wall surrounding the top of the hill can be accessed by the entrance at its northern side. Within the inner area of the hill, the church of St. George of Dorgana still exists, with ancient columns standing within its interior. At the northwestern side of the hill, crumbling remains of a settlement can be seen. According to legend, the castle was never penetrated, except on one occasion due to treason. Despite a path making it accessible, the castle sees few visitors due to its remote location.