The Torre del Fraile is one of a set of military watchtowers built around the South and East coast of Spain to keep an eye on passing shipping and Barbary pirates. The watchtowers were in sight of one another and it was therefore possible to get a signal to Gibraltar from the watchtower in Tarifa. The tower was designed by Luis Bravo and Juan Pedro Laguna in 1588.
The tower is about 240 metres back from the sea and Cala Arenas and 120 metres above it. The tower is over six metres in diameter and was over thirteen metres high until the top collapsed in 2006 with the loss of a window and the upper staircase. The tower's entrance was over five metres in the air and this led onto a floor. From this the soldiers could climb to the top where space was reserved for firewood for signalling.
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.