Vitrified forts also occur in Sweden, and in this context are subject of my own research into hillforts of the Migration period. The vitrification there is considered to be corresponding to a destruction phase, and is thus useful for thermoluminescence dating of these things, in correspondence with a C14 date from construction wood dating the construction. It has been suggested that these 'forts' are actually ritual enclosures analogous to Neolithic Henges, but this can not be the full explanation for their meaning, particularly during the later Migration period (though this might be true for most of the Bronze Age). However, ritual aspects definitely played a role in their prehistoric interpretation, so a manipulation by fire beyond any functional goal is plausible. On the other hand, I find it quite dubious that somehow, this ritual burning would have been a practice in so many different unconnected areas in Europe, throughout the Iron Age. The amount of wood that goes into the construction of even stone ramparts is quite considerable, particularly when dealing with cultures with no real drystone construction tradition (that is, lacking the Neolithic tradition of drystone tomb building, or the Bronze Age Nuraghes from Sardinia). Most ramparts/fort walls in Northern Europe are made of earth and palisades, and houses are similarly timber-supported. I suspect therefore that the amount of wood involved might have been enough for accidental vitrification during attack, or as a byproduct of deliberate destruction of the wall.