These represent only one of several types of defensive site in the north-east, dating from c 900BC to c AD 900. They are called 'vitrified' because the stones of their rampart walls have been fused together by intense heat, caused by the firing of a wall strengthened by beams of timber running through it.
Experiments have shown that a great deal of brushwood and other fuel is required to achieve the fusing (and, also, the collapse of the wall). The technique must therefore have been carried out some time after the fort had fallen to an enemy, perhaps as a form of psychological warfare.
The technique of timber-lacing is not precisely dated; it is found on (presumptively) early first millennium BC sites such as Dunnideer, and Tap o' Noth as well as on sites as late as the ninth century AD such as Green Castle, Portknockie, in Moray.