Board Thread:General Discussion/@comment-26006155-20190820122909/@comment-33883848-20190825154013

Jacky720 wrote: After the war, the monsters retreated to the end of the Underground- Home. After Chara fell, the friendly human child gave them the confidence to expand outward. So when New Home was built, they added a second bed. Trying to find a citation, I thought this was canon.

This is not confirmed but heavily implied.Though,that weird that monsters says that Asriel brought Chara to "the castle",i mean sure the Dreemurs could have a castle in the ruins but the wordering implies that there always existed only one castle.Perhaps,they think Frisk know about which castle they are talking about.