Maybe whatever fortifications were in place weren't enough for all the dunedain, in terms of strength or space of accomodation; in some circumstances a forest could provide a better cover than a fixed position, and some of the elder edain used forests as cover during the first age, IIRC. For the downs themselves, I don't think that, individually, they were sophisticated enough to protect their inhabitants in large numbers and for a long time against attempts to draw them out, it just wasn't what they were built for. And, to return to Tom Bombabil's words, it seems that the only times the tombs were disturbed was when the wights entered them.
Quote:
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There was victory and defeat; and towers fell, fortresses were burned, and flames went up into the sky. Gold was piled on the biers of dead kings and queens; and mounds covered them, and the stone doors were shut; and the grass grew over all. Sheep walked for a while biting the grass, but soon the hills were empty again. A shadow came out of dark places far away, and the bones were stirred in the mounds. Barrow-wights walked in the hollow places with a clink of rings on cold fingers, and gold chains in the wind.
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Edit:
It has also occured to me that, besides being unable to counter beyond a short term, attempts to "smoke 'em out", the graves could also be subject to flooding; and unless they are made of Orthanc-hard stone, they can be penetrated from above, sides, or one could simply reinforce the entrance, burry the dunedain for good, and forget about them. After all, what the drugs say about them, that they eat stone, shouldn't be taken literally
Edit II:
I just remembered the referrence in The Hobbit, Over Hill and Under Hill, that orcs always delighted in wheels, engines and explosives. I guess it would have been really fun for them to implode barrow after barrow
