I was reading a book that mentioned the war between Rome and Alba Longa early in it's existence, and how the Romans completely destroyed the city after their victory. Livy writes at the end of the first century, "Every building, public and private, was leveled with the ground. In a single hour the work of four hundred years lay in utter ruin." Although I'm sure this is an exaggeration, how would a conquering army go about razing a city of rock and stone to the ground in the shortest amount of time?
Actually most homes and buildings would have been primarily made of wood and there would likely be only a few stone buildings at all, especially during the time of Alba Longa since that precedes most of Rome's own great construction projects. Poor folk tended to live in homes made of mud bricks and wood, so burning down and collapsing a great majority of a city would actually be very easy. As for stone buildings, either man power or allowing it to decay would suffice. A stone tower in a pile of rubble, or a castle surrounded by flames, is of little value and will likely become a ruin.
Fire, for one.
Augustus famously left Rome a "City of Marble" by his death in 14 AD, but in 64 AD large portions of Rome were destroyed (while Nero fiddled) in the "Great Fire." Wood was still a huge part of their architecture, and this was WAY after Alba Longa was destroyed in the 7th century BC. Do you have a source claiming that it was "a city of rock and stone?"