Board Thread:Questions and Answers/@comment-5808186-20170801093304/@comment-27136653-20170809014721

After Chara came, monsters realized that humans are no longer a threat, seeing how good friends Chara and Asriel became. They've decided to move to a new home, which the king named "New Home". After the kids died and Asgore declared a war on humans, Toriel decided to move back to Home (the old capital). And that's where we find her in the game, along with a few other monsters that also decided to stay. And the reason why it's named "Ruins" is because, well, it IS ruined. The city's been abandoned for a long time.