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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

The barracks, designed by William Scamp (assistant director, Admiralty Works Department), were built as headquarters for the Royal Marine Artillery, who moved in in 1867. After the amalgamation of the Royal Marine Light Infantry and RM Artillery in 1927, Eastney served as headquarters for the Portsmouth Division of the Corps (which also maintained a Depot at Deal in Kent). The series of seven linked blocks facing the sea forms the second longest barracks frontage in the country (after the Royal Artillery Barracks, Woolwich).
At the same time as the barracks, a pair of small artillery forts were built on the foreshore. Eastney Fort East is still extant (having remained in military use until 1989); Eastney Fort West has been converted into a walled garden.
The barracks still stand, having been sold by the MOD in 1995 and converted into private dwellings. The ensemble has been called 'the best and most complete barracks of the post-Crimean War period'. One building (the former officers' mess) was retained and currently houses the Royal Marines Museum
Many of the buildings are Grade II listed. Link , Link & Link

In the United Kingdom, a scheduled monument is a 'nationally important' archaeological site or historic building, given protection against unauthorised change.
There are about 20,000 scheduled monuments in England representing about 37,000 heritage assets. Of the tens of thousands of scheduled monuments in the UK, most are inconspicuous archaeological sites, but some are large ruins.
Wikipedia: Link