The Fortress San João Baptista da Foz do Douro was built in the late sixteenth century to better protect the coast and the mouth of the Douro River. The monument is occupied today by the Regional Delegation of the Institute of National Defense. It is a fine example of military architecture. The original basic structure was enhanced with more recent bulwarks. The Lawn Tennis Club da Foz is located at the foot of the Fort.
Visits to the Fort are free on weekdays between 9h and 17h30. There are no official guided tours, but if you are part of a large group, it is advisable to contact the Fort ahead of time to ensure that the visit does not disturb its activities.
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.