To access Dolbury park at the Killerton House car park (see link below), go through the gardens around the front of the house, and follow the path upwards. Whilst climbing the hill passing the sequoias, there is clear view to the south of Woodbury and the nearby barrows distinctive on the horizon. You eventually get to the top of the formal garden and go through a gate in a metal railed fence.
From what I can recall from the information boards at the top, Dolbury is on the top of a volcanic outcrop, the inner embankments are older than the outer enclosure and the main entrance was on the northern side. The eastern side didn't have any embankments to protect it due to it's steepness. The site has never been excavated.
A possible origin of the dragon myth is that it was created to protect from looting sacrificial offerings made to its well. The existance of the dragon could never be disproved due to the fact that no matter which you visited, either Dolbury or Cadbury Castle, the dragon would invariably be at the other!
From Westcot's History of Devon.
Harl. MSS. No. 2307. (1630?)
If Cadburye-castle and Dolbury-hill dolven were,
All England might ploughe with a golden sheere.
Cadbury-castle, (alias Caderbyr) the land of William de Campo Arnulphi, and after of Willowby, Fursden, and now Carew. This castle may be seene farr offe (so they terame of highe upright, topped hill) by nature and slyght art anciently fortified, which, in those Roman or Saxon warrs, might be of goode strength, conteyninge within the compass thereof, near... acres. Here you may see some fyve mile distant, to the south-east, in the parish of Broad Clyet, another down, called Dolbury-hill:- between these two hills (you may be pleased to hear a pretty tale) that is said (I sett not downe those wordes to lessen your belief of the truthe of the matter) but to lett you know that, nil praeter auditum habeo:
Take yt on this condition
Yt holds credyt by tradition;
That a fiery dragon, or some ignis fatuus in such lykeness, hath bynne often seene to flye between these hills, komming from the one to the other in the night season; whereby it is supposed ther is a great treasure hydd in each of them, and that the dragon is the trusty treasurer and sure keeper thereof, as he was of the golden fleece in Cholcos, which Jason, by the help of Medea, brought thence; for, as Ovid sayth, he was very vigilant.
A watchfull dragon sett,
This golden fleece to keep,
Within whose careful eyes
Come never wink of sleep.
And, as the two relations may be as true one as the other, for any thinge I knowe, for it is constantly believed of the credulous heer, and some do averr to have seene yt lately. And of this hydden treasure the ryming proverbe here quoted goes commonly and anciently.
Quoted on p168 of A provincial glossary by Francis Grose, new edition 1811.