The servant was pretty detailed about how to get to Blackerly before she passed out. You guys are in room 11, in the south east corner of the keep's great hall. She told you that the poker game Blackerly was running is in the eastern gatehouse, which is room 3c. The only way in there is to go into the western gatehouse (room 3a), climb up the ladder into the upper gatehouse (3d), and then climb the ladder down into the eastern gatehouse (3c). From what she's told you, that's the only way in or out of it, and it will take you about 15 to 20 minutes to get there, moving carefully and stealthily.