The ancient city is located within the modern Turkish city of İznik (whose modern name derives from Nicaea's), and is situated in a fertile basin at the eastern end of Lake Ascanius, bounded by ranges of hills to the north and south. It is situated with its west wall rising from the lake itself, providing both protection from siege from that direction, as well as a source of supplies which would be difficult to cut off. The lake is large enough that it could not be blockaded from the land easily, and the city was large enough to make any attempt to reach the harbour from shore-based siege weapons very difficult.
The ancient city is surrounded on all sides by 5 kilometres (3 mi) of walls about 10 metres (33 ft) high. These are in turn surrounded by a double ditch on the land portions, and also included over 100 towers in various locations. Large gates on the three landbound sides of the walls provided the only entrance to the city.
Today the walls have been pierced in many places for roads, but
much of the early work survives and, as a result, it is a major tourist
The place is said to have been colonized by Bottiaeans, and to have originally borne the name of Ancore (Ἀγκόρη) or Helicore (Ἑλικόρη), or by soldiers of Alexander the Great's army who hailed from Nicaea in Locris, near Thermopylae. The later version however was not widespread even in Antiquity. Whatever the truth, the first Greek colony on the site was probably destroyed by the Mysians, and it fell to Antigonus I Monophthalmus, one of Alexander's successors (Diadochi) to refound the city ca. 315 BC as Antigoneia (Ἀντιγονεία) after himself. Antigonus is also known to have established Bottiaean soldiers in the vicinity, lending credence to the tradition about the city's founding by Bottiaeans. Following Antigonus' defeat and death at the Battle of Ipsus in 301 BC, the city was captured by Lysimachus, who renamed it Nicaea (Νίκαια, also transliterated as Nikaia or Nicæa; see also List of traditional Greek place names), in tribute to his wife Nicaea, who had recently died.
Sometime before 280 BC, the city came under the control of the local dynasty of the kings of Bithynia. This marks the beginning of its rise to prominence as a seat of the royal court, as well as of its rivalry with Nicomedia. The two cities' dispute over which one was the pre-eminent city (signified by the appellation metropolis) of Bithynia continued for centuries, and the 38th oration of Dio Chrysostom was expressly composed to settle the dispute. Plutarch, mentioned that Menecrates (Μενεκράτης) wrote about the history of the city.
Along with the rest of Bithynia, Nicaea came under the rule of the Roman Republic in 72 BC. The city remained one of the most important urban centres of Asia Minor throughout the Roman period, and continued its old competition with Nicomedia over pre-eminence and the location of the seat of the Roman governor of Bithynia et Pontus. The geographer Strabo (XII.565 ff.) described the city as built in the typical Hellenistic fashion with great regularity, in the form of a square, measuring 16 stadia in circumference, i.e. approx. 700 m × 700 m (2,297 ft × 2,297 ft) or 0.7 km × 0.7 km (0.43 mi × 0.43 mi) covering an area of some 50 ha (124 acres) or 0.5 km2 (0.2 sq mi); it had four gates, and all its streets intersected one another at right angles in accordance with the Hippodamian plan, so that from a monument in the centre all the four gates could be seen. This monument stood in the gymnasium, which was destroyed by fire but was restored with increased magnificence by Pliny the Younger, when he was governor there in the early 2nd century AD. In his writings Pliny makes frequent mention of Nicaea and its public buildings.
Emperor Hadrian visited the city in 123 AD after it had been severely damaged by an earthquake and began to rebuild it. The new city was enclosed by a polygonal wall of some 5 kilometres in length. Reconstruction was not completed until the 3rd century, and the new set of walls failed to save Nicaea from being sacked by the Goths in 258 AD. The numerous coins of Nicaea which still exist attest the interest taken in the city by the Roman emperors, as well as its attachment to the rulers; many of them commemorate great festivals celebrated there in honour of gods and emperors, as Olympia, Isthmia, Dionysia, Pythia, Commodia, Severia, Philadelphia, etc.