The Council was convened by Emperor Constantine I. After discussing with Church leaders he picked the town of Nicaea, present-day Iznik, Turkey to host the council.
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Many people assume Nicaea was chosen as the spot for the council because of its closeness to the new capital of the Roman Empire, Constantinople. However, Constantine did not refound the city of Byzantium as the capital city of Contantinople until 330. In fact, Constantine only united the Roman Empire in 324. Until then the Roman Empire was under a Tetrarchy where the four main political centers were Trier (Germany), Milan (Italy), Sirmium (Serbia), and Nicomedia (Turkey).
Nicaea was instead chosen because of religious geography. The Lost History of Christianity does a great job pointing out that most Christians lived in the eastern realms of the Roman Empire and beyond. Further, the Church was more organized in the "civilized" East with modern-day interior Turkey being known as the second holy land due to its many centers of religious learning. Outside of Italy much of the western parts of the Empire were either outpost cities or semi-wild lands. By having the council in Nicaea various Church leaders from Asia and Egypt could easily travel while Western bishops could still reach the city due to the extensive road and shipping networks.
Nicaea was still attended mostly by Greek-speaking Eastern Church members, as they were more numerous and closer, though important Western, Latin speaking Church leaders were still able to attend.
The bishops agreed on Christ's divinity and gave us the first form of the Nicaean Creed, the creed which is considered a standard of what all Christians believe.