ニカイア (Nicaea) is an ancient city located in present-day Turkey, near the modern city of İznik. It is historically significant for being the site of the First Council of Nicaea in 325 AD, which was convened by the Roman Emperor Constantine the Great to address various theological disputes within early Christianity. The city was also known for its beautiful lake, Lake İznik, and its impressive walls, which were built during the Byzantine period. Nicaea played a crucial role in the development of Christian doctrine and was a center of trade and culture in the region throughout its history.