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Sissinghurst Castle Garden is a National Trust owned property and one of the most famous gardens in England.
The Castle itself has changed many times over the centuries. Originally there was a large defensive stone castle (to the East of the current site), which was abandoned and a new building built adjacent. Much of that building was demolished centuries ago and now just the front section and a tower remain.
Sissinghurst was used as a farm for the last few centuries and an oast house and hay barn sit adjacent to the remaining buildings.

Listed buildings and structures are officially designated as being of special architectural, historical or cultural significance. There are over half a million listed structures in the United Kingdom, covered by around 375,000 listings.
Listed status is more commonly associated with buildings or groups of buildings, however it can cover many other structures, including bridges, headstones, steps, ponds, monuments, walls, phone boxes, wrecks, parks, and heritage sites, and in more recent times a road crossing (Abbey Road) and graffiti art (Banksy 'Spy-booth') have been included.

In England and Wales there are three main listing designations;
Grade I (2.5%) - exceptional interest, sometimes considered to be internationally important.
Grade II* (5.5%) - particularly important buildings of more than special interest.
Grade II (92%) - nationally important and of special interest.

There are also locally listed structures (at the discretion of local authorities) using A, B and C designations.

In Scotland three classifications are also used but the criteria are different. There are around 47,500 Listed buildings.
Category A (8%)- generally equivalent to Grade I and II* in England and Wales
Category B (51%)- this appears generally to cover the ground of Grade II, recognising national importance.
Category C (41%)- buildings of local importance, probably with some overlap with English Grade II.

In Northern Ireland the criteria are similar to Scotland, but the classifications are:
Grade A (2.3%)
Grade B+ (4.7%)
Grade B (93%)

Read more at Wikipedia Link

A National Trust is an organization dedicated to preserving the cultural or environmental treasures of a particular geographic region. They generally operate as private non-profit organizations, although some receive considerable support from their national government. The first such organization was the National Trust for Places of Historic Interest or Natural Beauty, which is the National Trust of England, Wales and Northern Ireland, formed in 1895 and operating as a charitable organisation.

Extract from Wikipedia Link

List of National Trust places Link