How does Alan arrange for David and himself to get out of Stirling? From chapters 25-30

They followed Alan Water until it flowed into the Forth and then hid close to Sterling Castle. David pressed for crossly the bridge directly but Alan was wary. There seemed to be no guard but Alan advised that they lay low. Later that night they watched a small woman waddle across the bridge. She went unbothered until suddenly a guard, likely awakened, stopped her. They had no chance to get across with the guard awake. Alan crawled further away from the bridge and David, forced to follow, saw his meeting with Mr. Rankeillor further postponed.