The Account of Daniel Defoe of his visit to Old and New Sarum in the 18thC
Daniel Defoe like John Leland two centuries before him actually visited the places he recorded. After novels like Robinson Crusoe and Moll Flanders, his three volume travel book, Tour Through the Whole Island of Great Britain was published between 1724 and 1727. His account of the Old and New Cities of Sarum appears in Letter 3 part 2
“………..we come to Salisbury; the vast flocks of sheep, which one every where sees upon these downs, and the great number of those flocks, is a sight truly worth observation; 'tis ordinary for these flocks to contain from 3 to 5000 in a flock; and several private farmers hereabouts have two or three such flocks.
But 'tis more remarkable still; how a great part of these downs comes by a new method of husbandry, to be not only made arable, which they never were in former days, but to bear excellent wheat, and great crops too, tho' otherwise poor barren land, and never known to our ancestors to be capable of any such thing; nay, they would perhaps have laugh'd at any one that would have gone about to plough up the wild downs and hills, where the sheep were wont to go: But experience has made the present age wiser, and more skilful in husbandry; for by only folding the sheep upon the plow'd lands, those lands, which otherwise are barren, and where the plow goes within three or four inches of the solid rock of chalk, are made fruitful, and bear very good wheat, as well as rye and barley: I shall say more of this when I come to speak of the same practice farther in the country.
As we pass'd this plain country, we saw a great many old camps, as well Roman as British, and several remains of the ancient inhabitants of this kingdom, and of their wars, battles, entrenchments, encampments, buildings, and other fortifications, which are indeed very agreeable to a traveller, that has read any thing of the history of the country. Old Sarum is as remarkable as any of these, where there is a double entrenchment, with a deep graffe, or ditch, to either of them; the area about 100 yards in diameter, taking in the whole crown of the hill, and thereby rendering the ascent very difficult: Near this, there is one farm house, which is all the remains I could see of any town in or near the place, for the encampment has no resemblance of a-town; and yet this is called the borough of Old Sarum, and sends two members to Parliament, who, those members can justly say, they represent, would be hard for them to answer.
Some will have it, that the old city of Sorbiodunum, or Salisbury, stood here, and was afterwards, for I know not what reasons, remov'd to the low marshy grounds, among the rivers, where it now stands: But as I see no authority for it, other than mere tradition, I believe my share of it, and take it ad referendum.
Salisbury itself is indeed a large and pleasant city; tho' I do not think it at all the pleasanter for that which they boast so much of; namely, the water running thro' the middle of every street, or that it adds any thing to the beauty of the place, but just the contrary; it keeps the streets always dirty, full of wet and filth, and weeds, even in the middle of summer.
The city of Salisbury has two remarkable manufactures carried on in it, and which employ the poor of great part of the country round; namely, fine flannels, and long cloths for the Turkey trade, call'd Salisbury Whites: The people of Salisbury are gay and rich, and have a flourishing trade; and there is a great deal of good manners and good company among them; I mean, among the citizens, besides what is found among the gentlemen; for there are many good families in Salisbury, besides the citizens.” 
Folding of sheep means moving them from pastures by day to manure arable land at night. This is a key part of bedwork or floated meadows and the sheep corn cycle farming system. He describes this as new but it was in use locally in the century before. It grew in popularity in the 18th century as, although it was labour intensive in moving the sheep, the rewards and increased productivity, of the sediment, trace elements, nutrients, aeration, temperature were great and could be moved from the grasses of the meadows to arable crops and so onto horses, cattle and humans.
Graff(e) is a ditch, trench, or canal used in fortification especially as a moat and etymologically linked to grave.
As he describes a single occupied building, the “farm house” it must have been the Old Castle Inn. It may have been called the Old Castle House then but it is thought an inn had stood on that site since the 12thC  and that he took refreshments there. 
ad referendum means for referral or subject to agreement
Turkey trade meant trade with the Levant and the Ottoman market. Woollens were shipped out by the Turkey merchants. The poultry bird acquired its name from them.
“Romans” is used very loosely as referring to something that was very old, there being little concept of cultures that long predated the Romans. It was known that they built temples, forts and racecourses and that they came to Britain so was not unreasonable to assume that Old Sarum was a Roman fort, Stonehenge a temple and the cursus a racetrack.
 Daniel Defoe “Tour Through the Whole Island of Great Britain” 1724-27 Letter 3 part 2
 Historic England https://historicengland.org.uk/listing/the-list/list-entry/1258431
 'Stratford-sub-Castle', in A History of the County of Wiltshire: Volume 6, ed. Elizabeth Crittall (London, 1962), pp. 199-213. British History Online http://www.british-history.ac.uk/vch/wilts/vol6/pp199-213 [accessed 27 September 2020].
Further reading on the village website / history section
Local History Group