İznik is a town and an administrative district in the Province of Bursa, Turkey. It was historically known as Nicaea, from which its modern name also derives. The town lies in a fertile basin at the eastern end of Lake İznik, bounded by ranges of hills to the north and south. The town is only 90 kilometres southeast of Istanbul but by road it is 200 km around the Gulf of İzmit. It is 80 km by road from Bursa.
The town 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 reach the harbour from shore-based siege weapons very difficult.
The city was surrounded on all sides by 5 km of walls about 10 m high. These were 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.