Late 80s early 90s dimension travel flick

hope someone can help me identify this film.

I've seen it sometimes in the 90s and from what I remember it looked like an old eighties/early nineties production. There were at least two teenagers, a boy and a girl, that were traveling through different dimensions. At one point, they are in space, fighting a sort of alien creature, after which the space ship explodes and they travel to a sort of medieval arthurian land, where the king is being visited by a noble that they identify as the villain and I think the wizard tells them how to travel through time. Near the end, I remember that the girl is tempted to dimension warp again, but sees that the next dimension is a sort of dark city ruled by biker gangs, and they decide against it.

That's pretty much all I remember. It would be great if somebody could figure it out. Thanks in advance.