Most of these last stand cities tend to be in terrain that makes it difficult to assail on foot. Transylvito is utterly inaccessible. Faq is in a remote mountain pass. Gobwin's Knob is on a volcano. Charlescomm is apparently near the top of the world. However, it also makes it difficult to flee on foot, if not impossible during a siege. (And what do Barbarians do, anyway?)

Where does it say that Unaroyal wasn't a helm's deep? One way in, one way out.