The Walls look like a casting of a hand sculpted master and the buildings look scratchbuilt or again a casting from an original master. Could well have been an in-house project within GW to make them.
There are several ranges of 1/600 (3mm) scenery around now that would make it easier to do today. For completeness you will still need to source the old Mighty Empires metal bits though which will not be easy.,