The Castle of Monção was built by King Denis of Portugal in 1306 and it was enlarged in 1656. From the original castle, the only remains are two doors, some pieces of the medieval wall, and the Torre de Lapela Towers, which shows off the coast of arms of King Ferdinand I.
From the walls, the only remain is the main door, defended by the Keep, with a Gothic style; and the treachery door, with a smaller size. From the 17th century fortress there are many parts still standing, such as the Porta de Salvaterra Door and some bastions.
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.