If anything, the Mayfair Ruins exemplify exactly why there are so many hillside fortresses in the Fallen Empire. During the ages of magical and traditional warfare between the Kale and their old servant races, most of the strongholds of both sides were sacked on at least one occasion, if not more.
However, while destroying a stone fort can be accomplished with a bit of hard work, it takes a lot more effort to destroy the subterranean structures built under and behind them. And if the invading forces couldn’t hold the land, these fortifications could be brought back into use with very little work and often became the homes of insurgent groups of both peasant militia as well as covens of spellcasters and clerics seeking otherworldly aid against the Kale.
Thus today there are hundreds of these fortifications attached to old ruins and new fortresses throughout the lands.