IN the 18th century, people expected a ruin to look picturesque, with plenty of ivy swagging the vacant windows, and a tangle of weeds and briars draped over mounds of fallen masonry. Ruins were privately owned and tastefully neglected, to look like ruins.

But, without upkeep, a ruin will not last long in this romantic state. Nature, left to itself, will steadily reduce it to a level where nobody can see anything but lumps in the ground.

The ruins of Slingsby Castle have been halfway to that condition ever since I have known them. Notices warn you not to try to go inside the walls. I would never dream of doing so, nor do I know anyone who has ventured against the warning. It's obvious that, apart from the danger of falling masonry, the jungle of trees and brambles could soon bring you crashing to the ground.

Yet as castles go, it isn't so very old, dating back only to the time of the Civil War.

There were castles there before that, the first built in the 14th century by Sir Ralph Hastings. In the 15th century, his grandson built a new castle on the same site. He ended beheaded by Richard III. 100 years later, a Sir John Atherton owned the land, and sold it to Sir Charles Cavendish.

Sir Charles was the youngest son of the famous Bess of Hardwick, at one time co-jailer of Mary Queen of Scots. Four times married and widowed, she had acquired with each marriage more lands and castles, until she owned not only Chatsworth but large tracts of Derbyshire and Nottinghamshire. Some of that property was inherited by Sir Charles.

Bess and all her descendants were great builders, Bess herself being responsible for Hardwick Hall, in its time and to this day, a showpiece of architecture.

In the year that Sir Charles bought Slingsby Manor, the second of his two sons, another Charles, was born. It was this Cavendish, grandson of Bess of Hardwick, who was responsible for the building of Slingsby Castle.

He was growing up at the time of King James I. And he seems to have been well-known and well-liked by his contemporaries, for his character rather than for his looks. John Aubrey, the famous diarist, described him as "a little, weak, crooked man".

Another contemporary wrote: "In this unhandsome and homely habitation was lodged a mind and soul that was very lovely and beautiful. Nature not having adapted him for the court or the camp, he betook himself to the study of mathematics, wherein he became a great master."

He may not have been "adapted for the court", but somehow he seems to have taken quite an active part in it. He was only 18 when, with his elder brother William, he was one of a mission party sent to Savoy by the King to discuss a marriage for his son and heir, Henry, who, however, died young, before the marriage had been arranged.

Charles Cavendish was still only 23 when his father died, leaving him a good estate, the income from which he mostly spent on books and learning. He was in touch with some of the best minds of this time, including Hobbes and Descartes, and the playwright Ben Jonson.

He still kept in touch with the court. At the age of 25, he was knighted by King James (admittedly not a great distinction: the King made 111 peers during his reign).

At 30, he was elected to the House of Commons. That was not an onerous duty. In those days, parliament was only sporadically summoned, usually when the king felt in need of a grant. In the time of King Charles I, of course, this became all too frequent, leading eventually to the Civil War in 1641.

Until then, Charles Cavendish had been able to live quietly enough according to his tastes, with his books and studies. And it must have been during this period, before the Civil War, that the building of Slingsby Castle began.

The architect he employed was called John Smythson, son of Robert Smythson, who had worked for his father and grandmother, and had been behind the building of Hardwick Hall. Slingsby Castle is said to bear a marked resemblance to Hardwick in its style.

But before the roof had been put on, the Civil War broke into Charles' quiet life, as into the lives of so many others in that time. He was to play a full part in the actual warfare, on the Royalist side. Despite his "stunted body", someone wrote, he "charged the enemy in all battles, with as keen a courage as could dwell in the heart of man".

But it was his brother, William, who was the outstanding Royalist general. Charles' fortunes were closely linked with his. After four years of hard fighting at William's side, at the battle of Marston Moor in 1644, the tide finally turned against the Royalists.

In the company of other losers, including Sir Hugh Cholmley, who had had to surrender Scarborough Castle, the brothers took refuge abroad. For some years, they lived in Paris, often on credit. They were both on the parliamentary list of "unforgivable enemies".

When Charles I was finally beheaded in 1649, all the Cavendish estates were sequestered by the Roundheads. Charles, however, was able to "compound" his estates, that is, pay a fine of £5,000.

He went to England in 1651 to settle this, and to try to claim a wife's allowance for his brother William's wife, Margaret. They had just enough money for the fare to London, but were at their wits' end to find enough for lodgings. Margaret did not find success, but Charles' case was settled. Land was sold, beneath its true value, to pay the fine. He was left with a fairly comfortable but very much reduced income. But, in three more years, he was dead.

Slingsby Castle had to be abandoned, still roofless. It was really never anything but a ruin. Wind and rain set to work to render it more so, and have been doing so for 350 years now.

In the 18th century, a vicar took stone from it to build his vicarage. No doubt he would not be the only vandal. It is surprising that any of it is still there at all.

Updated: 14:47 Wednesday, June 04, 2003