[@Guardian Angel Haruki]
I think technology would be limited to medieval era, as I assume it is when humans fought monsters and forced them underground, with evidence being the pictures in the opening slideshow

As for Chara and Asriel I think that they came in a couple hundred years after the monsters were sealed. Where Asgore and Toriel wear clothes that look medieval-ish, while Asriel wears clothes that look more modern. 

Even so, I think we should try and steer away from including the main Undertale characters, though we could mention or reference Asgore and Toriel since theyre the king & queen of monsters :)