Rangrim the chest was mostly emptied before the keep was abandoned, but you find a secret compartment in its lid. This compartment contains a wooden scroll tube, inside which is a scrap of paper with the following words written on it in Common:
Wooden lance, Moonlight dance, Model keep, Cistern deep, Below the shield, By chains concealed
Alex studies the paper for a moment, "I concur. It appears to be exactly what we need." He stops for a moment longer. "Then we best be on our guard, and keep an eye out for access to a sewer and the dungeons." He handed the paper back to Grim. "We should possibly rest up before venturing below. I would imagine more undead and possibly traps stand in our way."
Ramgrim takes the key and turns it in the lock and the door swings open. The battlements to the West and some stairs leading down into the courtyard are revealed. The storm rages on.
From where you all stand you can see that if you go down the stairs, you will be down in the Courtyard again. If you follow the battlements you can trace them all the way around to the front gatehouse.