Mount Grace Priory, Yorks.

English Heritage
In a grassy space are substantial ruins of the small priory church, and of the four-room cells inhabited by the monks. One of the cells has been restored and furnished to show how the monks lived. Each Carthusian monk mostly lived alone in his cell, to which meals were brought. Each cell had its own garden, and its own running water and primitive toilet.
An interesting site, worth a visit if you are in the area.

Helmsley Castle, Yorks.

English Heritage.
The castle, originally a medaeval fortress, later acquired a luxurious Tudor house within its walls. The castle was rendered unusable at the end of the Civil War. Later still, it was regarded as a Victorian romantic ruin.
Today, the circuit of walls is largely intact. Inside is a wide area of grassy lawn. The visitor enters via a gatehouse and can look at the half of the keep still standing, the other half having been blown up and allowed to fall into the moat. The Tudor residence is now used as an exhibition space. An audio guide is available.
The castle is definitely worth a visit if you are in the area. Other EH properties are nearby.

Hardwick Old Hall, Derbyshire

Old Hall, trees English Heritage
The old hall was remodelled by Elisabeth Shrewsbury before she had the new hall built. The roofless ruins still stand to almost their original height, and visitors can ascend four floors to view surviving decorative plasterwork. Kitchens and service rooms are also visible. The house was partly dismantled in the 1750’s. An audio tour is available, and there is an exhibition in the West Lodge describing Bess’s adventures in architecture.

Fountains Abbey, Yorks.

National Trust & English Heritage
Fountains Abbey is now rated as a World Heritage Site. The abbey ruins are unusually complete, with many parts standing at near their original height. Perhaps the site was too remote to be extensively pillaged for building stone. There is much to see and do on site. Besides exploring the extensive abbey ruins, you can explore the beautifully landscaped Georgian water garden of Studley Royal, and visit Elizabethan Fountains Hall, the Cistercian monastery corn mill, and St Mary’s Church.
The gardens have a lake and many temples and follies, and even a twisting tunnel designed to give visitors a mild scare. If you are a fan of William Burges, do not miss the Victorian church, which looks plain on the outside, but inside is a riot of Victorian Gothic colour and carved ornamentation in wood and stone.
You should plan for an all-day visit.

Cliffords Tower, York

The tower comprises an unusual quatrefoil shell wall, built on top of a high mound. It’s almost all that remains of York Castle. Visitors can climb to the top of the wall, which affords great views over the central parts of York. The middle of the tower, which may once have been roofed, is now an open courtyard. If I remember correctly, there are a few relics and exhibits to look at inside the tower.
If you’re disabled, check photos of the mound and tower before planning your visit 🙁
There is a pay car park nearby, but you probably don’t want to be driving around the centre of York during the day, and you may find access on foot or by bus less troublesome.

Audley End, Essex

House front English Heritage
In contrast to many EH properties, Audley End is complete and fully furnished. The house origins date from the period of Henry VIII, when Audley was three times its present size, but the present appearance of the house, inside and out, dates from the 19th century. Noted occupants include Henrietta Howard, later a royal mistress and occupant of Marble Hill House, Richmond. The contents, including pictures and a natural history collection, are well worth seeing. Note that most of the best stuff is on the first floor, so that on our last family visit my sister and I saw a lot more items than did my late mother, who was in her wheelchair and preferred to wait below. Apart from the house, there is the Service Wing, with re-enactments of servant life, The Stable Block, with horses, the Walled Garden, the Gardens with artificial lake, and extensive parkland. The gardens behind the house are worth a walk-around.
Recent revisit: August 2018, nurseries now open on upper floor.
Suggested visit time: at least a half day – there’s plenty to see and do.

Stables
Stables
garden front
Garden front
Temple of Concord
Temple of Concord

Old Gorhambury House, Herts

Remains of Old Gorhambury House
Remains of Old Gorhambury House

This was once an immense Elizabethan mansion, but was largely dismantled when the present Gorhambury House was built, the remains being left as a ready-made romantic ruin in the grounds.
Though the site can be visited at any reasonable time, access is awkward, as it is in the middle of a private estate and only foot access is permitted. I got to it by parking on the A4147 near the M1 & M10, and near the entry to Beechtree Lane, and making the vexatiously long walk along the permitted path to the house. (The other end of the “Gorhambury Walk” permitted path is at the Roman Amphitheatre in St Albans). If you visit Gorhambury House by car (on a Thursday afternoon only, via St. Albans) you can reach the old house by a much shorter but still significant walk, starting from the gardens at the rear. You might need directions, as there is no clear line of sight between the old and new houses, the view being blocked by trees. The ruins don’t show up well on an aerial view, but it looks like if you start at the back of the mansion you should walk away from it bearing slightly to your left. The drive continues past the old house, but the roadway is very narrow, and there is nowhere to park.
Is it worth it? Once you get there, it’s quite an interesting historic ruin, but won’t detain you for long.

View of Gorhambury Mansion
Gorhambury mansion seen from near old House ruins

Houghton House, Bedfordshire

English Heritage
The shell of a once imposing 17th century mansion, which was stripped by its owners when no longer required. The site commands some fine views (possibly why they built there). Parking is nearby, at the end of a long access roadway. An interesting visit, but probably won’t detain you very long. A free audio guide can be downloaded from the English Heritage website.