San Nicolás de la Villa is one of the twelve Fernandine churches built in the city after its conquest by King Ferdinand III of Castile (1236), in Gothic-Mudéjar style, although it was completed only in the following centuries, with much renovation added in the meantime. The main portal, in Renaissance style, was added by Hernán Ruiz the Younger in the 16th century. The octagonal tower, with a defensive appearance, was rebuilt during the age of the Catholic Monarchs above an older Islamic minaret.
In the 17th and 18th century, much of the church's elements were modified until it received the current mostly Baroque appearance. The baptistery dates from 1555.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.