The thing is, a lot mid to late Iron Age hill forts used timber lacing to give the wall flexibility against battering rams. Caesar mentions this in The Conquest of Gaul and say how the trick was in the length of the timber lacing, he found the longer ones almost impossible to penetrate. So A) You'd want the flexibility timber gives.
B) if this was a common technique, why aren't there vitrified forts elsewhere. (even in non rocky areas, during the building of the ditch, those Iron Age guys have been known to carry on into the rock to quarry out stone for the ramparts).
Also, in order to burn wood needs air, hill forts where extremely well built by expert stone masons with tight fitting seams, I think if a fire were to break out the worst you'd get inside the wall would be charcoal.

A related and interesting fact is the speed that a fort could be built. Caesar mentions the Gaulish Celts building a fort with six foot (I think) high earthern Rampart with ditch to a circuference of 15 roman miles overnight.

Stanwick, with its stone faced ramparts of six miles circumference and mammoth dimensioned wall (10-15m from botton of ditch to top of wall) was probably built in a year.

I'm an expert of Internet connections that don't work properly! have you tried a different service provider? I think Microsoft may be up to its tricks again with Win 98 and Netscape, so try Internet Explorer for your browser. Who is your Internet Service Provider? I work in the industry and may be able to advise (but don't expect me to be objective!).