The fortified town of Rello still maintains the original defensive walls and castle that were used as protection against civilisations that came from the south following the Douro River. It is the best-preserved walled enclosure in the province.
You can go inside through either one of the two gates located on the sides of the castle. The castle is protected by a fortified enclosure with circular and quadrangular towers that have elbow openings. The upper part of the towers are crowned with 15th-century machicolations and you can still see some remains of the keep, a water pool and a wall of the gate that separated the castle from the rest of the town.
In the outer enclosure, there are still gunboats in the lower section that were used for artillery.
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.