Defence With its enormous wall mass - the plinth is even solid in some cases - the tower offered passive protection for the areas of the castle behind it. For this reason, at many castles the bergfried was sited on the main avenue of attack, often set into the front defensive wall. Thus, the bergfried was able to perform a similar function to the shield wall. This was particularly the case with castles in which shield wall and bergfried were interconnected to form a single structural unit (e.g. Liebenzell Castle in the Black Forest). So-called 'double bergfrieds' like that of the Greifenstein in Hesse and Rochlitz Castle in Saxony in a sense represent an intermediate stage between a bergfried and a shield wall The two closely spaced towers are linked by a narrow section of shield wall. That bergfrieds with pentagonal or triangular plans are mostly aligned with a corner facing the main line of attack, is also associated with the shield function: stone projectiles hurled by catapults were deflected laterally by the oblique angle of impact. In some cases, such "deflection wedges" (Prallkeile) were also added to the tower later, and they can even be found on towers with an otherwise circular plan (e.g. the Zvíkov Castle in Bohemia and Forchtenstein Castle in Austria). A square bergfried set up on a corner could also serve this purpose. In other cases, the acute-angled floor plan is, however, simply due to the natural shape of the bedrock Observation Because the bergfried was the highest building in the castle, it usually functioned as watchtower or observation tower. From the top storey or the fighting platform the foreground and the area surrounding the castle could be observed. Watchmen (Türmer) could give early warning of an approaching enemy, raising the alarm. During sieges the raised observation post was important for observing the foreground. A particularly well preserved example is the Osterburg in Weida: under the masonry spire of the bergfried is a watchman's residence and just under the spire is a small, original stone observation platform for the watchmen at a height of almost 58 metres above the ground. Raised fighting platform An enemy attacking a spur castle or hillside castle could often position himself above the castle itself. The height of the bergfried could at least in some cases compensate for that. From the elevated fighting platform the hillside could be better controlled