Quote Originally Posted by Yanagi View Post
I would argue for purposes of that last question--where would there be abandonned castles--that fortresses and forts should be considered in addition to castles, because as purely-strategic constructions they're much more likely to let go because their military purpose has expired; they have no political-status value such that they'd persist as a posh residence; and they're more likely to be sited in some distant, isolated location that only became important as a zone of control during a past conflict.
Castles tend to be located along strategic waterways. Waterway position is not static. Consequently, abandoned castles are likely to be located in the high and dry after rivers have moved, harbors have silted up, lakes have gone dry, or the like. Many Central Asian fortifications were rendered uninhabitable after the Mongols destroyed the networks of irrigation canals that made the surrounding regions agriculturally viable.

One useful option provided by fantasy in this vein is that you can have some alteration, climate change, brutal invasion, magical cataclysm, etc. that renders an area uninhabitable for one species, but perfectly acceptable to another. Hypothetical example, if elves are dependent upon forest agriculture, and the 'Rain of the Fiery Stars' turns all the region's forests into grassland, their might be tons of elven ruins, including viable fortifications, in a region well suited to colonization by humans.