This actually plays into a theory I've had about the origins of the WoIaF - if you think of the world as a far future version of ours, with Essos as what has become of Europe (post millenia of tectonic activity), then Asshai is the equivalent to Iran or Pakistan. Now, as most of us are aware, Asshai is filled with twisted, horrible mutants and people with strange powers. It is also barren of most plant life except for glowing grasses. So I'm thinking (along the lines of this theory) it was the site of a long since forgotten nuclear blast (or series of them) that were part of a war that set humanity back thousands of years technologically. As far as I understand, all of the magic and mysticism (with the exception of the Children and the Others) originates from here, so it could be loosely attributed to major genetic interruptions of all the flora and fauna in the region based on the intense radiation left from the series of blasts.

The tremendous engineering of the dawn age (just after the rising sun -- giant nuclear explosion anyone?) can be potentially explained by survivors with specific technical know-how from before this theoretical apocalypse... due to the intensely small number of actual survivors and amount of time that has passed since the beginning of their history it makes sense to me that there would be some capable of rallying the force to build what they had planned, but not to educate enough people to pass on the knowledge effectively and kick start the redevelopment of a "modern" society - at least in Westeros.

In Essos, my theory is that the Valyrians (relatively geographically and onomatopoetically close to Iberians) were able to preserve mining technologies and perhaps some kind of geothermal venting in order to access the wealth of ore in the supervolcano in which they chose to house their Freehold and to prevent said supervolcano from erupting - or that was the plan. Generations of misunderstanding and lore made people look at it as magic as opposed to science - this in turn led to imprecision and improper educational practices eventually leading to the venting systems being overwhelmed due to lack of proper maintenance, and in the end, the Doom.

Now, this is all just a theory that I came up with while reading AsoIaF and aWoIaF -- i've no textual citations or any real evidence, but i think it would be fuckin cool if it were the case.