Stagoi (modern Kalabaka) was located on the site of ancient Aiginion, a stronghold (phrourion or kastron) and bishopric in Thessaly known from the 10th century onward. According to an act of 1163, Stagoi belonged to the theme of Servia. This act gives a list of the properties of the bishopric (many villages having Slavic names) and exempts the bishop's klerikoparoikoi from diverse levies. John VI Kantakouzenos names Stagoi among phrouria that had belonged to the Gabrielopoulos but in 1333 were occupied by John Orsini of Epiros. From the mid-14th century all of Thessaly was controlled by Stefan Urog IV Dugan, and Serbian kephalai administered Stagoi. Its bishop was suffragan of Larissa. The first monasteries at Meteora were apparently under the bishop's control, and his rights are confirmed in imperial rescripts of 1336 and 1393 preserved on the walls of the cathedral. The stronghold and the bishopric, however, soon declined and fell under the domination of either the monks or the bishops of Trikkala. Several monuments are known to have existed in Stagoi, among them a church dedicated to St. Barbara, but of these only the cathedral, dedicated to the Dormition of the Virgin (Koimesis Theotokou), survives. This is a three-aisled basilica, constructed probably in the late 11th or early 12th century on the foundations of a church from the 4th-6th century, the (rebuilt) ambo, chancel screen, and synthronon of the earlier structure survive in the interior, and there are mosaics under the pavement of the floor. Some late 12th century frescoes (standing portraits of saints) remain in the south aisle, although most of the decoration is from the latter part of the 16th century.