One of the "Must See" attractions of Dubrovnik is the fortification walls surrounding the old city. While there were wooden fortifications here dating back to the 7th century, the stone walls, considered to be one of the great fortification systems of the Middle Ages, were constructed between the 12th and 17th centuries ( with the bulk of the numerous additions and modifications occurring between the beginning of the 15th century and the latter half of the 16th century). These fortifications hold the reputation of never having been breeched by hostile armies. Running uninterrupted for 1940 meters (6360 feet), and reaching a maximum height of 25 meters (82 feet), the walls contain 3 circular and 14 quadrangular towers, five bastions (bulwarks), two angular fortifications, and the large St. John's fortress. There actually are four strong fortresses situated at significant points along the walls; to the north is the circular Minceta tower; to the east, the Revelin fortress; on the west is Fort Bokor ( the old town main entrance and considered the oldest preserved fort of its kind in Europe); and on the southeast is the large St. John fortress complex. In 1979, this fortification complex was added to the UNESCO World Heitage list. While some of the brochures state one can tour the walls in 90 minutes, we found that three hours was required to really explore the whole site, including the number of museums contained within the complex.