The city lies in a fertile basin at the eastern end of the Ascanion Lake, 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 cannot be blockaded from the land easily, and the city was large enough to make any attempt to interdict the boats from shore-based siege weapons very difficult.
The city is surrounded on all sides by 5 km of walls about 10 m high. These are in turn surrounded by a double ditch on the land portions, and also include over 100 towers in various locations. Large gates on the three landbound sides of the walls provide the only entrance to the city.
Today the walls are pierced in many places for roads, but much of the early work survives and as a result it is a major tourist destination. The town has a population of about 15,000.
Nicaea (Greek Nikaia) was originally founded around 310 BC by the Macedonian king Antigonus, who had taken control of much of Asia Minor upon the death of Alexander the Great (under whom he served as a general). The city was at that time named after him, Antigoneia. Several other of Alexander generals (known together as the Diadochi) later conspired to remove Antigonus, and after defeating him the area was given to Thessalian general Lysimachus (Lysimakhos) (circa 355 BC-281 BC) in 301 BC as his share of the lands. He renamed it Nicaea, in tribute to his wife Nikaia.
The city was built on an important crossroads between Galatia and Phrygia, and thus saw steady trade. It appears to have lost some of its importance during the early Roman era a few hundred years later, but this changed dramatically with the split of the empire into west and east. The eastern half, later known as the Byzantine Empire, made the city an important defensive stop to the south of Byzantium (Constantinople). Much of the existing architecture and defensive works date to this time, early A.D. 300s. Major earthquakes struck in 358, 362 and 368, ruining much of the early city's architecture.
It was during this time that the early Christian leaders met in Nicaea in 325 under the direction of Constantine, in what would later be known as the First Council of Nicaea, the first of many Ecumenical councils. They met to consider the issues of Arianism, resulting in the current concept of the Trinity and the creation of the Nicene Creed. The church of Haghia Sophia was built by Justinian I in the middle of the city in the 6th century (modelled after the larger Hagia Sophia in Constantinople), and it was there that the Second Council of Nicaea met in 787 to discuss the issues of iconography.
The city saw a long period of peace under Byzantine rule, which lasted until the rise of the Seljuk Turks. In 1077 they took the city, which exchanged hands several times in the next year until it was firmly in their control by 1078. Here they formed their capital.
This event was instrumental in starting the First Crusade at Byzantium's request in 1095, and armies from Europe along with smaller units from Byzantium converged on the city in 1097. After the European armies laid siege to the city and penetrated the walls, they were surprised to awake the next morning to see the Greek flags of Emperor Alexius I flying over the city. Robbed of their chance to plunder the city, the crusaders and Byzantines were soon at odds.
Constantinople later fell in 1204 to the European armies in the Fourth Crusade, who set up the Latin Empire of Constantinople. They had poor control over the area, and a number of Byzantine successor states sprung up as well, including Epirus and Trebizond. However it was Nicaea that formed the core of the successor Byzantine Empire after Theodore Lascaris (who became Theodore I) founded the Empire of Nicaea there. Theodore I and his successors slowly expanded their domains, and in 1259 Michael VIII Palaeologus usurped the throne. He captured Constantinople from the Latins in 1261, and restored the Byzantine Empire.
In 1331 the city fell to the Ottoman Turks, who renamed it Iznik. With the fall of Constantinople in 1453, the town fell in importance, but later became a major center with the creation of a local faience pottery-making industry in the 17th century. However this industry also moved to Istanbul, and it became a minor town in the area when a major railway bypassed it.