Shouldn't the demon side's greatest advantage be that they don't actually have any solid place to defend? They might have some locations, otherwise they wouldn't be able to survive so easily, but they shouldn't be compared to cities with history. After all, the demons are a destroyed nation. Fugitives that have been left alone for ambiguous reasons. The humans, however, won't be able to send out all of their forces because they always need to leave behind enough to defend their cities. The demons could just carry their meager possessions to another place. All the bullshit talk aside, this might be the major reason why there still are demons alive, after Rei got sealed and the initial genocide stopped.