Locating a small removable brick, you remove it and find behind it a small knob, the secret door opens into a staircase that leads to another hidden door, opening the door up the stairs you find yourselves in the burned black chamber up in the ground level, the most north eastern part of the ruined keep. (The square room north east of the main hall of the Moathouse)
Thandis didn't find any mechanism, so no disabling can be done to the portcullis, but with the appropriate strength, you guess the bars can be lifted.