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DUMPY, could it be Deep Underground Military Position Yellow? or could it be named after a piece of engineering equipment - a dumpy level? Whatever it stands for the level was called Dumpy from the very begining when it was built as a combined operations floor following the failure of Bastion (which may or may not be there). As stated on TV last evening, Dumpy was used during the Cuba crisis, as a RSG or Regional Seat of Goverment, many of which were built all around the country.
There are indeed a few more underground complexs under the castle, upper and lower Hudsons, Long Gun Magazine to name but three. I have been into Dumpy more times then I can remember and I agree with Paul Wells, it is boring, it was abandoned halfway through a major refit so little is left to see. £6 for a trip, well, you pay your money and you make your choice, all I can say is that when tours were first mooted (years ago) £20+ was talked about!