Nicaea (Νίκαια, modern İznik), a Bithynian city located on Lake Ascanius, was one of the most important Byzantine cities. It served as the capital of the empire in exile and was the seat of two ecumenical councils.

It prospered from its location on major trade and military routes and its control of a fertile region. In late antiquity, it was a large, powerfully fortified city filled with civic and private buildings laid out on a regular plan. It was a major military base—site of the proclamation of Valens as emperor and of the revolt of Prokopios - and seat of an imperial treasury where tax revenues were deposited. Earthquakes in 363 and 368 combined with the growth of Constantinople provoked decline, but were later rebuilt by Justinian I. During these centuries, the church of Nicaea flourished: Valens made it a metropolis independent of its ancient rival Nicomedia, conflicts between the two sees flared at the Council of Chalcedon, originally planned to meet in Nicaea. After a period of obscurity, Nicaea frequently appears in the 8th century and later as a powerful fortress. In 715, it was the refuge for Emperor Anastasius II, and in 716 and 727 it resisted Arab attack. It was a major bulwark on the highway that led to Constantinople. Damage from the siege of 727 was compounded by an earthquake in 740. Nicaea, base for the revolt of Artabasdos, became capital of Opsician theme in the 8th C. In the 10th C., Nicaea was a center of administration and trade, with a Jewish community and an imperial xenodocheion. Rebels sought to control it as a strong point near Constantinople: Bardas Skleros, Isaac I Komnenos, Nikephoros III Botaneiates and Nikephoros Melissenos all fought in and around Nicaea. When Melissenos joined Alexios I in the West in 1081, he left Nicaea to his Turkish allies, who soon assumed control. Nicaea was thus capital of the first Turkish state in Asia Minor until the First Crusade captured it in 1097 after a long siege, their first victory in Asia and the only time in history that Nicaea succumbed to direct assault rather than blockade. Alexios I took control of Nicaea from the reluctant Crusaders and defended it against the Turks. In 1147, Nicaea was the supply base for the abortive Second Crusade and in 1187 unsuccessfully revolted against Andronikos I.

After the fall of Constantinople in 1204, Nicaea at first took an independent position, but recognized Theodore I Laskaris in 1206; he was crowned there in 1208. From that date until 1261 Nicaea served as capital of the empire, although John III Vatatzes  resided in Nymphaion and Magnesia; it was also the seat of the patriarch and home of many illustrious refugees, notably Niketas Choniates, Nicholas Mesarites, and Nikephoros Blemmydes. Laskarid Nicaea was the scene of frequent synods, embassies, and imperial weddings and funerals and became a center of education, notably under Theodore II Laskaris, who founded and endowed an imperial school. After the recapture of Constantinople, Nicaea declined in importance and prosperity. Neglect of the eastern frontier provoked a serious revolt in the region in 1262, and in 1265 the whole city panicked on rumor of a Mongol attack. In 1290 Andronikos II arrived on a tour of inspection and restored the walls, but the region remained defenseless against a new foe, Osman. Nicaea held out until 1331, when it fell to the Ottomans after a long blockade. When Gregory Palamas visited Nicaea in 1354, its Christian population was severely depleted.

The well-preserved walls of Nicaea, completed in 270, manifest numerous styles of construction representing constant rebuilding, notably in the 8th, 9th, 12th, and 13th century. Originally a single rampart 5 km long with So towers, built of rubble and brick, the walls were raised and strengthened before being transformed by John III, who added an outer wall and a moat. The most noted of Nicaea's churches was the monastery of Hyakinthos, known in modern times as the Church of Koimesis. A rectangular structure with a cruciform nave surmounted by a dome on massive pillars and separated from the aisles by arcades, it manifests affinities with a group of cross-domed basilicas and appears to date to the late 6th century. The church was decorated with mosaics whose images, replaced by the Iconoclasts, were restored after 843. It was rebuilt and redecorated after the earthquake of 1065 and stood until 1924. The surviving basilica of Hagia Sophia in the center of the city, probably site of the council of 787, preserves traces of its elaborate marble decoration. Most renowned in the 13th century was the Church of St. Tryphon, which possibly was recently discovered. Two 13th-century other churches, known as “Church B” and “Church C”, have not been identified. Civic buildings have not been preserved, with the exception of the Roman theater, abandoned and used as a quarry and dump after the 7th century. The 13th-century city is known in some detail, from the enkomia of Theodore Laskaris, delivered before John III ca.1250, and of Theodore Metochites, addressed to Andronikos II in 1290. Although the speeches are filled with extravagant rhetoric, they give an image of the city in its regional context and show that churches, monasteries, charitable institutions, palaces, and houses shared the area within the walls with extensive open spaces.

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