storage buildings. The main tower of a castle was often simply referred to as a "tower" (Turm) or "big tower" (großer Turm). In late medieval Low German documents, however, the terms berchfrit, berchvrede and similar variants often appeared often in connexion with smaller castles. German castle research during the 19th century introduced Bergfried or Berchfrit as the general term for a non-residential main tower, and these terms then became established in the literature. The etymological origin of the word is unclear. There are theories about it being derived from Middle High German or Latin, or even from a Greek word brought back from the Crusades. A theory that is often stated in older texts, that the bergfried took its name from the phrase "weil er den Frieden berge" ("because it keeps the peace"), i.e. it guaranteed the security of the castle, cannot be confirmed. Development and forms The bergfried established itself as a new type of building during the 12th century and from about 1180 to the 14th century increasingly became a feature of the Central European castles. Numerous examples have survived from this period almost to their full height. However, the origin of the design is not fully understood, since towers dating from before the 12th century have had to be almost entirely excavated archaeologically, and only the lowest sections remain. Individual examples (like the bergfried of Habsburg Castle) may also be found dating to as early as the second half of the 11th century. The precursor of the bergfried is the fortified tower house, whose Western European expression is called a donjon or keep. Residential towers were common before the advent of the bergfried in German-speaking countries, too; a precursor is found, for example, in the wooden tower of the motte-and-bailey castle. Donjons combine the two contrasting functions of a stately, comfortable residence and a fortification. The bergfried, however, dispenses with the keep's residential function in favour of its defensive purposes. At the same time, new forms of unfortified residential building became popular, the palas, for example, was incorporated into castle construction. The emergence of the bergfried is thus clearly related to the differentiation of living and fortification within a castle. In Western Europe however, the donjon or keep, with their combination of domestic and defensive functions, continued to be predominant during the course of the Middle Ages. Often the bergfried forms the main tower in the centre