Fehtija Mosque is a mosque and former Catholic church located in the town of Bihać, Bosnia and Herzegovina. Built in 1266, it is the oldest gothic building in the country. It was originally built as a Catholic church dedicated to Saint Anthony of Padua, and was subsequently transformed into a mosque following the 1592 conquest of Bihać from Habsburg Croatia by the Ottomans. The building was originally accompanied by a monastery, which was also mentioned in a 13th-century charter of the Croatian nobility.
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.