There are two access quests within Netktropos Castle in addition to the Lockets Quest. One is "The Boar's Head" and one is "The Red Marble". Check out
the quest listing for Nektropos Castle here at AllaK's to find them. Finishing The Boar's Head will give you access to the Barracks which contains a chest which is the quest starter for The Red Marble. Finishing The Red Marble gives you access to the Library.
HTH