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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 Barbican is the name given to the western and northern sides of the old harbour area (Sutton Harbour) of Plymouth. It was one of the few parts of the city to escape most of the destruction of The Blitz during the Second World War. Two or three streets still retain some of the architecture of an old fishing port.
The present Barbican district is generally regarded as being roughly equivalent to the location and size of the medieval walled town of Sutton. A barbican is a fortified gate, and here the name probably derives from the 'Castle Barbican' which was an entrance to the late medieval fortress that guarded access to the Cattewater, prior to the building of the Royal Citadel.
For centuries the Barbican was home to Plymouth's fish market (now relocated to the other side of the harbour) and is still home to many fishermen.
Today the Barbican is mostly pubs, bars, cafes and restaurants.