As with so many Fortean subjects, the common reaction to the mystery is often as fascinating as the mystery itself.
When you read about vitrified forts, you could be forgiven for imagining walls that had been fused into a single continuous, rather lumpy piece of glass with the shapes of the original rocks being barely discernible. How could this be caused by anything less than lasers, or a nuclear attack? How indeed?
However, a bit of simple searching shows that the vitrification was patchy, ranging from entire lengths of wall to isolated sections, small groups of rocks, and so on. Sometimes, rocks are fused together and at other times, only glazed or partially melted.
The four causes to consider are:
1) Extreme natural causes, such as comet strikes. This can be excluded simply because there are so many examples, so widely dispersed. What natural cause could be so extreme, and so comparatively common (over 100 instances) and yet not obvious to us? What natural cause would only occur over a period of a few centuries? Where are the vitrified ancient Greek, or Roman, high mediaeval, or Tudor structures, for example? What natural cause would only affect built structures but not the scattered natural rocks in the same area? No, we can reasonably exclude this.
2) Accidents, such as accidental fires. Many of the same considerations apply. Why did they only have such accidents in such a small window of time and place?
3) Deliberate enemy action in an attack. This could be subdivided into (3a) rival tribes using conventional iron age/early mediaeval techniques, and (3b) some previously unknown technological superpower.
3a) Rival tribes. There would be no obvious advantage in vitrifying an enemy's fort in the course of an attack. In iron age and early mediaeval times, long sieges were probably rare because of the logistical problems and the the absence of standing armies. If a fort were to be attacked at all then subterfuge or direct assault would be likely, and any use of fire would probably be focussed either in the area of the intended assault, or in one other place as a distraction. Piling wood, brush or peat, or similar combustible substances against the wall would be difficult if the defenders were armed with slings, arrows or javelins. It seems unlikely.
3b) Unknown technological superpower (including space aliens). Firstly: why use atom bombs or lasers on a primitive garrison of iron age fort dwellers? If you can make atom bombs or lasers, you can probably make helicopter gun ships, machine guns, armoured fighting vehicles, mortars, or any of a hundred other comparatively simple pieces of hardware that would do the job more efficiently. Lasers famously fire in straight lines, so why use them to fuse the defences, rather than picking off individual guards or key timber buildings? Finally, where is the other evidence of this technological superpower? Where are their own buildings, their roads, their artefacts? As Sagan said, extraordinary claims require extraordinary evidence, and as Occam's razor instructs us, why make these extra assumptions rather than working with the evidence we already have? I think all these "unknown superpower" explanations should be dismissed until we have a credible case for a particular technological superpower having existed at the time.
4) Deliberate act for cultural reasons. This could break down into (4a) by the constructors, and (4b) by victorious enemies.
Whichever version of (4) we consider, the explanation has to lie in this general area. The phenomenon is comparatively limited geographically (primarily Scotland, but also some other parts of Britain and western Europe) and is also limited in terms of period: iron age and early mediaeval — or, as we used to say, "the dark ages".
So there was something cultural going on in that region in that period.
People have been sceptical about it being for defensive purposes because vitrification "weakens the rock". However, in that period, no one was using artillery to smash the rock. Any attempt to breach the walls would probably be by digging at them, and rocks that are bonded together would be harder to remove than loosely piled dry stone walling. However, I favour the idea that if a fort were to be attacked, it would probably be either by subterfuge or by direct assault, rather than siegecraft.
We could of course hide behind the archaeologist's traditional catch all of "ritual significance". The vitrification could have been to sanctify the fort, or purge it of spirits, or to cleanse it, or to neutralise it at the end of its useful life. We have examples of other old structures that were systematically destroyed, or filled in, as if the intention was to dispel something: memories, associations, evil spirits?
A simpler explanation may simply be status and aesthetics. When we see hill forts today, we see misshapen banks and ditches and rocky or grassy enclosures. These once thriving places are now often bleak. In use, the banks and ditches may well have been well kept, and perhaps faced with timbers or stones, maybe whitewashed, or deliberately blackened with soot. Perhaps thorny plants such as gorse were encouraged to grow in the ditches or on the outside faces of the banks as a natural way of slowing down attackers — with the additional benefit of gorse flowers being attractive. The banks may well have had timber palisades and maybe even lengths of timber rampart, isolated watch towers, and a timber gate house straddling the main entrance. Any and all of these may have been decorated with banners, coloured or painted hides, and so on. Against this background, partial vitrification of the stones to create a particular visual effect may have been no more than a fashionable statement of power and prestige.
I am no expert, but reading on the subject tells me that temperatures of 1,200 degrees would have been sufficient to vitrify some of the rock. Iron oxide can produce iron at 1,250 degrees, so in the iron age, they at least had the capability to produce temperatures this high. After that, it becomes merely a question of scale, determination and technique.
While we sit at our desks wondering how "the ancients" built Stonehenge, or Avebury, or the pyramids, or the vitrified forts, without modern technology, they just got on with it. If you have enough people, enough time, enough will to succeed, and a strong leader with access to the skills, then no special technology is required.