Nicaea (also spelled Nicæa or Nicea, /naɪˈsiːə/ ny-SEE-ə;^[9] Latin: [niːˈkae̯.a]), also known as Nikaia (Greek: Νίκαια, Attic: [nǐːkai̯a], Koine: [ˈnikεa]), was an ancient Greek city in the north-western Anatolian region of Bithynia^[4][10][11] that is primarily known as the site of the First and Second Councils of Nicaea (the first and seventh Ecumenical councils in the early history of the Christian Church), the Nicene Creed (which comes from the First Council), and as the capital city of the Empire of Nicaea following the Fourth Crusade in 1204, until the recapture of Constantinople by the Byzantines in 1261. Nicaea was also the capital of the Ottomans from 1331 to 1335.

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; as a result, it is a tourist destination.