The living quarters of a castle with a bergfried are separate, often in a lower tower or an adjacent building called a palas (an English-style keep combines both functions of habitation and defence.) Consequently, a bergfried could be built as a tall slender tower with little internal room, few vaults and few if any windows. The bergfried served as a watchtower and as a refuge during sieges (at least if the siege was relatively brief). The distinction between a bergfried and a keep is not always clear-cut, as there were thousands of such towers built with many variations. There are some French keeps with only austere living quarters, while some late bergfrieds in Germany were intended to be habitable (Piper 1900).
For maximum protection, the bergfried could be sited on its own in the centre of the castle's inner bailey and totally separate from the enceinte. Alternatively, it could be close to or up against the outer curtain wall on the most vulnerable side as an additional defence, or project from the wall. For instance, the Marksburg has its bergfried in the centre, and Katz Castle on the most likely direction of attack. Some, like Münzenberg and Plesse Castles, have two bergfrieds.
Outside Germany, the crusader castles of Montfort Castle and Khirbat Jiddin built by the Teutonic Order had prominent towers that some authors have compared to bergfrieds (Kennedy 2000, Folda 2005), arguing that these castles depended more on Rhineland than local crusader traditions of military architecture.
The word '"bergfried", sometimes rendered perfrit, berchfrit or berfride and many similar variants in medieval documents, did not just refer to a castle tower, but was used to describe most other types of tower, such as siege towers, bell towers (c.f. its cognate belfried or belfry) or 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.