I haven't read that particular book I'm afraid, but if I remember correctly, it was the siege of Caen where the undesirables were expelled and forced to live between between the besiegers and the besieged. There are chronicles and sources describing that particular event happening. I'm not sure if its 'common', but it certainly could and did happen. It's interesting because Henry is not condemned for his actions of not permitting the civilians to cross his siege lines, he had absolutely no obligation to them. From the English viewpoint it was Caen's duty to protect its inhabitants, or surrender the city to the English if they couldn't. Indeed, leaving them there, suffering in view of the defenders, was probably a calculated move to some extent.