ISAURIA. – It is the westernmost province of the patriarchate of Antioch and, in the ecclesiastical geography of the Byzantine period, comprises only part of the former Roman province, roughly the valleys of the Lamos and the Kalykandos and the mountainous massif between the latter and the sea. Its archaeological exploration is incomplete. The metropolis of I. is Seleucia, near which arose the most important sanctuary dedicated to the cult of St. Tecla, which had supplanted that of the demon Sarpedon and the goddess Athena; it consisted of a martyrium and monasteries enclosed within a fortified wall to protect against brigand attacks. The sanctuary attracted a very large number of pilgrims from every region. The miracles of St. Tecla also mention a chapel of the Saint at Selinunte, built on an acropolis and often defending the city against enemy attacks: another monastery of St. Tecla, known through an inscription, existed at Diocesarea, and a sanctuary at Dalisandos, which was a pilgrimage site almost as famous as that near Seleucia. The mountainous character of the region protected the Christian communities during the first centuries of the Muslim invasion, so that representatives of the ancient bishoprics still appear, in part, at the Seventh Council, while Giorgio di Cipro, the Nea Tactica and the Notitia Antiochena provide as many as eight names of episcopal sees.
The most important group of monuments in I. is that of the sanctuary of St. Tecla near Seleucia, in the locality now called Meriamilk: a large three-aisled basilica divided by columns, probably built in the second half of the 5th century and altered at a later date; a cave beneath it transformed into a church; a domed basilica over the central nave, probably a foundation of the Emperor Zeno (574–91); and, somewhat farther away, the “North Church.” Of the greatest importance is this domed basilica, which documents a new conception of architectural space in the evolution of basilical forms. In Seleucia, an ancient Roman temple had been transformed into a church by replacing one of the short sides with an apse. The basilica of St. Tecla is notable not only for its great dimensions, but principally because the two rooms flanking the apse, in the Syrian manner, are each provided with an apse, according to a structure characteristic of central and northern Syria (the cathedral of Il-Änderin, the martyrion and Basilica B of Rusäfah).
At the northern end of the Kalykandos valley, near the village of Hoğa Kalessi, stands another extremely important church: a three-aisled basilica of perfect construction in squared stone, with a technique (including the decoration) distinctly Syrian. The central nave is divided into two parts by transverse arches, and above the part in front of the presbytery rises a tower whose upper storey, above the matroneum, has chamfered corners with four niche-like connections supported by columns, similar to those of the side chambers beside the apse of the basilica of St. Sergius at Rusäfah and of the Red Monastery near Sähäg, all monuments dating from the middle of the 5th century. Also distinctly Syrian in plan, with the apse flanked by two chambers, are the large matroneum church of Gâmbazli and one at Tapureli; by contrast, one church at Olba and two at Sök Örem (Korassion) are related to structures widespread in Cilicia, namely, structures having a room behind the apse, in the first case itself provided with two apses. Its liturgical function is unknown. One church at Olba and one at Tapureli have only the diaconicum. All these three-aisled basilicas are provided with a narthex; at Meriamille, both in the domed basilica and in the North Church, there is also an atrium, evidence of the penetration of Hellenistic elements into a region that may be regarded as the extreme northwestern extension of Syrian architecture.