Nicaea

Nicaea, also known as Nikaia, was an ancient Greek city in the north-western Anatolian region of Bithynia that is primarily known as the site of the First and Second Councils of Nicaea, the Nicene Creed, 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, 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.

Source: Wikipedia