CCN usage is a smart way to scale the castle's difficulty, especially if players may travel through them out of order. The consistent castle layout is helpful and very Super Mario. The Silverfish's transform mechanic is a neat way to "promote" a mob monster.
I'd agree with the other comment that the sheer amount of enemies may overwhelm Shadowdark characters. Shuffling in unique treasures or giving the transformed commoners personalities might enrich a castle's unique feel.
Fun concept and very NES-style!