This is why I think PoR especially missed a great opportunity: Imagine if the portrait levels were to have played out in classic, linear, bottomless-pits CV formula. It would have totally gelled because the main castle retains the Castletroid mechanics, while the portrait levels feel like separate entities (which they should, because they are).
Not unlike Mario 64, if you would die in these paintings, you'd be kicked out of it and have to start over.
Woulda made that game way more interesting and challenging.
There would of course have to be multiple paths and bosses, like in Rondo
