vassals in times of battle. The basements of bergfrieds were often embedded several metres into the ground. Undermining was therefore not a great threat. Arson was also very difficult due to their stone construction and the few light openings could be quickly closed to prevent being smoked out. The conservative historian therefore sees the bergfried as a means of passive defence, as a refuge for a few days until relief arrived. For this reason, very few facilities for active defence can be found in these buildings. The main aim was to prevent an attacker breaking in. To storm such a tower within a few days is almost impossible. Thanks to their solid construction many bergfrieds even escaped later demolition attempts by the surrounding rural population, who wanted to carry off building materials from abandoned castles and reuse them. An attack on such a fortified site within an active feudal system was almost hopeless. It was far less risky simply to plunder the farms and mills of the enemy. In fact, a large number of Central European castles were never seriously attacked during the Middle Ages. Consequently, there is not much evidence of a retreat into a bergfried; the building had already fulfilled its deterrent function. A siege was only worth undertaking if the attacker had previously ensured he had legal authority and had asked the state sovereign or even the emperor for permission. This was only possible where there had been actual or fabricated violations of the law, such as highway robbery, forgery or murder. The hands of those who had sworn allegiance to the sovereign were then tied; for legal reasons they could not come to the aid of the attacked lord. In such cases, taking final refuge in the main tower was almost pointless. The bergfrieds of 12th and 13th century castles were originally surrounded only by simple defensive walls. Flanking towers and zwingers were added in later phases. Many outbuildings were then made of wood or were half-timbered, and stone housing was usually not fortified. In the Middle Ages in the event of a siege, a massive bergfried was undoubtedly the safest building in which women, the elderly and children could seek refuge during the fighting. Such a tower was certainly an effective protection against surprise attacks by smaller marauding gangs and the local population. Often a castle was vulnerable just through the absence of a few