(sector 224) - above the library. The door switch is behind a curtain in the library.
(sector 234) - access from north end of hallway behind the red door.
(sector 284) - in the middle of hallway, behind a figure.
(sector 322) - a mancubus needs to be killed to enable tag 666 and open secret door in the NW corner of the castle. Get the radsuit first!
On the easiest skill setting the level does not have any Mancubi, thus the 666 tag -- which requires all the Mancubi on the level to be killed -- never triggers. Because of this the walls to the secret chamber never open, making it impossible to get 100% secrets on the easiest setting.