It’s always exciting when a “new” National Trust house opens. Chastleton House, on the border of Oxfordshire and Gloucestershire, is interesting and unusual. It has modest grounds - just a few acres - no grand approach, no elegant park, and is in the heart of the village. And the family who owned Chastleton for nearly four centuries changed it very little.

What the Trust received from its last owner, Barbara Clutton-Brock, in 1991 was a striking piece of Jacobean architecture which, because the family was never particularly wealthy, had become gradually run down. It had been opened to the public in the 1940s when the then owner, Irene Whitmore-Jones, would tell visitors that her family had lost its money “in the war”. She meant the Civil War 300 years earlier. This lack of change gave the house a special quality which the Trust was anxious to preserve. So in preparing Chastleton for the public it adopted a policy of “minimal disturbance”. The result is a refreshingly “lived in” house.

Nobody knows who designed the elegant symmetrical facade and the tall, compact, outward-looking shape, but the chances are it was somebody distinguished. The house is built around a small central courtyard, which means there are few passages and mostly the visitor passes from room to room. But there are two fine staircases leading to the huge gallery on the top floor, which runs the length of the house.

It is quite possible that Walter Jones, the first occupier, had a hand in designing Chastleton. He had acquired the land from Robert Catesby, one of the Gunpowder plotters, who had lived for some time in an earlier house on the same site. Jones was a lawyer from South Wales who was transforming himself into a country gentleman, so he wanted an up-to-date country house.

When Chastleton was finished in 1611, Sir Walter would have come in through a fine wood-panelled hall where servants would eat their meals and where the squire would meet his tenants. (Gradually the Jones family acquired much of Chastleton village, which surrounds the house.) On the ground floor were parlours, one of which has been in continuous use as a dining room since the house was built. Today it is laid out for a simple meal at an oak refectory table. The chairs have a dusty, battered look, but a magnificent tapestry from Lille in northern France and a fine ceiling frieze add a touch of distinction.

The most important rooms are on the first floor. The Great Chamber was used for dancing and concerts. There are fine bedrooms, many decorated in a way that showed Walter Jones’s clear desire to impress and to marry his children into the gentry. One bedroom, the Cavalier Room, has a marvellous romantic story associated with it. During the English Civil War the Jones family was on the King’s side.

After being defeated at the Battle of Worcester, Walter’s grandson Arthur Jones, “the Cavalier”, fled to Chastleton where he hid in a tiny, concealed room entered by a secret door in his own bedroom. When the Roundhead cavalry arrived they found Arthur’s exhausted horse and knew he wasn’t far away.

They demanded supper and lodging from Arthur’s young wife Sarah, and decided to sleep in the first room they came to - Arthur’s bedroom. But the wily Sarah spiked the Roundheads’ beer with laudanum and they fell into such a drugged sleep that Arthur was able to come out through the secret door and escape. Although he survived, the family’s fortunes never really recovered from the fines imposed on them by Cromwell.

On the floor above the bedrooms is the dramatic Long Gallery with its 72ft barrel-vaulted ceiling. This was where the Jones family would exercise, playing battledore (an ancestor of badminton) and perhaps doing some rustic dancing. The room is certainly large enough to do some serious jogging and has a magnificent view over the gardens with their circular topiary and the rolling countryside beyond.

The kitchens are in the basement and have been, in estate-agent language, in “urgent need of modernisation” for about 300 years. They are largely in their original 17th-century condition and include the last word in “bacon flakes” - ceiling racks for storing hams. There is a beer-cellar and a prodigiously long but now rotten ladder which was used throughout the 19th century to clear the gutters. In 1854 Dorothy Whitmore-Jones wrote in her diary, “Gore came to repair the upper windows and the great ladder was reared”. A few years later her son Walter gave the family an odd modern claim to fame: he codified the rules of croquet and won the first English Championship in 1868.

Repairs have been kept to the minimum and there has been very little modification. Only Walter Jones-Whitmore’s croquet lawn has been re-created. Otherwise the Trust has cleverly allowed the home of a family which has been neither particularly distinguished nor very rich to speak for itself.

Chastleton House, Moreton-in-Marsh, Oxfordshire GL56 OSU. Reopens in April. Tel: 01608 674 284. Visits must be pre-booked