Monastery of San Juan de Duero, built in the Romanesque style, consists of a single nave with a wooden roof, semicircular apse, and a pointed barrel vault. From the 12th century it belonged to the Knights Hospitaller of Jerusalem, until it was abandoned in the 18th century. The 12th century church and the 13th cloister, with Gothic and Mudéjar elements, are still standing.The arcades combine the various architectural styles current in Spain at the time: late Romanesque, early Gothic, and especially, Islamic tracery. These arches are criss-crossed to achieve a beautiful visual effect. Inside the church there is a display of items from Jewish, Islamic and Christian cultures.
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.