The brigands had 2 rounds of surprise, which they used to wedge closed the door to the room where the players were. Then they grabbed the weapons looted from their previously-defeated comrades (rescuing one who was tied up by the PCs) and took off, closing the portcullis on the way out. (They didn't know about their leader's secret cache, or they might have stayed to fight for it). I need to roll later to see if they grabbed a few of their prisoners on the way out, but the chances are low....

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