The location seen in this flashback is the vaults beneath Viktor Grafeneck’s castle–Rosemary’s place of origin. There is a detailed overview of the history between Rosemary and Milo on Westward’s Patreon page.
A big thanks to Matthias, the latest reader to become a Pioneer by supporting Westward on Patreon! Every hour spent producing Westward is an hour not spent earning money to support my family by other means. Supporters like Matthias recognize that Westward can only continue and grow if value is assigned to it by those who enjoy it. Thanks for believing, Pioneer!