of the castle or is positioned as a wall tower on the main avenue of attack against the castle (especially in the case of spur castles). It may be an isolated structure standing alone amongst the other buildings of the castle or be joined to them to form a combined building complex. However, typically, the bergfried is a self-contained element that is not internally connected to other buildings and has its own access. As a rule, this is a so-called elevated entrance, i.e. the entrance is located at the level of an upper floor of the tower and is accessed via its own bridge, staircase or ladder. Bergfrieds very often have a square or round floor plan, but pentagonal towers are also frequently encountered; whilst octagonal towers are rather less common. There are even a few examples of bergfrieds with irregular polygonal floor plans. A rare form is the triangular bergfried of Grenzau Castle near Höhr-Grenzhausen or that of Rauheneck Castle near Baden bei Wien. Towers with triangular and pentagonal floor plans invariably had a corner facing the main line of attack on the castle. Bergfrieds averaged 20 to 30 metres in height, although those at Forchtenstein Castle in Burgenland, Austria, and Freistadt Castle reach a height of 50 metres. Compared with the donjon, which occupies a relatively large ground area because of its elaborate interior layout with living rooms, a hall, kitchen, etc., the bergfried usually has a much smaller footprint, which, although of similar height to the donjon gives it the slimmer appearance of a tower. As a building, the bergfried has an even stronger vertical emphasis than the donjon. Local rock was usually used for building material, and was quarried in the immediate vicinity of the castle site. In areas where there was little usable rock, brick or fieldstone was used. The masonry work was often executed very carefully, edges being accentuated with rusticated ashlar. The bergfried could be plastered or the stonework could be left exposed. The latter was the case, for example, in the towers of the Hohenstaufen era which were entirely made of rusticated ashlar. The tower shaft (i.e. the main part of the tower between the base and the top floor) usually had no or very few windows; where they exist they are often just a few narrow vertical slits. The enormous wall thickness at basement level in many bergfrieds usually decreases significantly