The Romans made great efforts to protect this city, which they named Nicea. In order to protect the city of Nicea, which was subjected to various attacks from these war raids,they built stronger walls, which were started during the time of the Kingdom of Bithynia but were damaged by earthquakes. The 4970-meter-long city walls surrounding the city in the form of a pentagonal polygon were strengthened during the Roman, Byzantine and Ottoman periods. You can see the traces of every civilization on the city walls. Since each civilization used the stones of the previous civilization, there is almost an intertwined history on the walls.

All four main gates are magnificent like triumphal arch, and Three of these castle gates have survived until today. It is seen that marble frieze pieces were used in Lefke Gate. İstanbul Gate is the most spectacular gate on the road to Constantinople. It was made more spectacular with the mask stones brought from the Roman Theater. Yenişehir Gate is partially standing. The Lake Gate, on the other hand, was completely overthrown.

İznik Fortress, which has 12 secondary gates and 114 towers built 10-15 meters apart, reveals the construction techniques of the war and defense strategies of the period.