''How was The Invisible Woman being held prisoner by Dr. Doom? I'm prepared to believe she was incapable of escaping like the others, I just can't figure out what hold he had on her.
Yeah, that bugged me too. I guess she was imprisoned in some sort of a cage in Doom's bedroom.
It looks like some sort of glass tube given the way it shatters. I guess Doom also had some way to make it soundproof as well. When you think about it, that is a fairly clever way to torment her: unlike the others, she is put in a spot where there will be people around her. And nobody notices her.
It's also located in his bed chambers; there's a panel where Doom preens in front of it, naked, after having had sex with the Black Widow. The implication is that he forces her to watch.