Iznik is a city in Turkey which is known primarily as the site of the First and Second Councils of Nicaea, the first two Ecumenical councils in the early history of the Christian church.

The city lies in a fertile basin at the eastern end of the Lake İznik (Turkish: İznik Golu), 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.