In legend, the God Dionysos was Nicaea's founder; according to record, some inhabitants of a small town of the same name near Thermopylai may have colonized it (Nonnus Dion. 15.170; 16.403-5; Dio Chrys. Or. 39.1 & 8). Moreover, in that locality is noted an ancient military camp of Bottiei; and the city was named Elikore, Ankore, when in 316 B.C. Antigonos Monophthalmos founded Antigoneia there (Strab. Geogr. 12.565; Eust. Il. 2.863). After the battle of Issos, in 301 B.C., Lysimachos conquered the city and refounded it with the name of his wife, daughter of Antipater. In 282-81 B.C. Nicaea came under the rulers of Bithynia and regained great importance (App. Mith. 6 & 77). It was only in 72 B.C. that Bithynia came under Roman domination at the conclusion of the Mithridatic war (App. BCiv. 5.139.1). Embellished under Augustus to the point of contending with Nicomedia for the seat of the provincial governor, Nicaea became the first city of the eparchy under Claudius, as we know from the coinage. Pliny the Younger, governor under Trajan, further enlarged the city. Hadrian visited Nicaea in 123 and undertook works of fortification that were finished in the 3d c. A.D. under Claudius II (Gothicus), after the Goths had already caused serious damage to the city in 258. Constantine continued the work of embellishment of his predecessors, and held the first council at Nicaea in 325. Justinian took particular interest in the city (Amm. Marc., 220.127.116.11; 2.2; 22.9.5), which was again chosen in 787 for the second council.
The geographical situation of Nicaea was particularly fortunate (Plin. HN 6.34.217; Strab. Geogr. 2.134; Ptol. Geogr. 5.1.3). Its position on the shore of Lake Ascania (Iznik Golu), on level and fertile ground, with wide roads for traffic that radiated from the city, made Nicaea a great Hellenistic center. Strabo minutely described the foundation of the new Lysimachan city: It had a square plan 700 m to a side; the roads were arranged with perpendicular axes, following the perfect regularity of the rectangular scheme; two large arteries crossed at right angles at the center of the inhabited area; the extensions of the roads led to the four gates of the city, visible from a fixed stone placed at the center of the gymnasium, a building that thus must be supposed at the heart of the urban plan.
The following monuments are listed by written history and inscriptions: a theater, a Sanctuary of the goddess Roma and of Caesar (built under Augustus), an Apolloneion, a market (built under Hadrian), an aqueduct, and churches and a palace erected by Justinian (Procop. De aed. 5.3). The coinage, from the period of Marcus Aurelius onward, commemorates a number of other monuments, among them the temples of Asklepios, of Dionysos, and of Tyche. The theater was to the SW of the city, though little remains of the building itself. Its recognizable dimensions reach a maximum of 85 x 55 m, and only part of the cavea is conserved; the orchestra and the skene have been lost. Its plan must have been Hellenistic but has been repeatedly modified (Plin. Ep. 10.48). A curious monument, the obelisk of C. Cassius Philieus, rises barely outside Nicaea on the road to Nicomedia, and must have been a family tomb. The obelisk, triangular in section, is 12 m tall, and is placed on a rectangular base 2 x 3 m. The Byzantine city, which rendered unrecognizable with its new constructions the ancient Nicaea, overlaid the Hellenistic-Roman city plan. The imposing earlier walls had by the 5th c. A.D. already undergone major renovation. This Byzantine construction has two aspects. The gates, with triple openings, and several towers, seem still to follow the Roman plan; but often the superstructures are Byzantine, and the definitive system is Turkish. The principal churches of Nicaea included the Cathedral of Haghia Sophia, originally a basilica with three aisles of the 5th c., that underwent repeated restoration until the 14th c.; and the Church of the Dormition of the Virgin, whose controversial chronology varies between the 6th-7th and the 8th-9th c., with the earlier more probable. Of notable interest were the rich mosaics of the cupola and the narthex, destroyed during the Graeco-Turkish War, known only from photographs and watercolors made at the beginning of this century. (N. BONACASA)
Info: Princeton Encyclopedia
(Princeton Encyclopedia of Classical Sites, from Perseus Project)