Don't you need to buy some jerky and give it to Marle's dad to piss him off and have him lock himself in his room...? Or have you done that? Or is that another sidequest? x_x

Edit: I lied, go talk to the king in 1000 AD with Marle in your party and he'll explain that he's being framed. Once that happens, you can go to the new section of the castle and you should be good from there.