San Nicolás church was built in the 12th century not only with religious functions, but also as a defensive bastion of the quarter during the strife with the neighbouring ones of St. Saturnino and Navarrería. The military role is evident in the watch tower. In 1222 the original Romanesque church was destroyed by a fire, and was replaced by a new building, consecrated in 1231.
The interior is in Gothic style, from different phases. It houses a Baroque organ. Exteriorly, of the Gothic period only two portal are visible today, as well as the apse. The corner portico was designed by Angel Goicoechea and built by a local contractor Blas Morte in the 1880s.
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.