Leave the castle through the back way instead of through the front door

The exit is through the cloister gate.  This is jammed, and has to be blasted open with explosives - the famous Sunburst device!

The key to the room containing the Sunburst device is in the most southerly room in the basement, close to the wall between the middle and right-hand tombs.

The room that holds the Sunburst device is on the South-East side of the Ground floor, near the chapel.  Leave the stair area via the East door, turn South and  then East;  the Sunburst room door is on your right.

When you have the device, go out to the North-West corner of the cemetery.  Place the device against the cloister gate, retreat to a safe distance and set off the explosion with a fire arrow.  Run through the gate - Mission complete!