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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

Canterbury was surrounded by a wall in Roman times. Traces survive here and there. A fragment of the Roman Queningate can be seen in the city wall opposite St. Augustine's Great Gate, and further up, nearer Burgate, the Roman foundation of the wall is visible.

The walls are mentioned in several Anglo-Saxon documents. In 1011 the Danes succeeded in breaking into the city, slaughtering the inhabitants, and tossing them over the walls.

It has not yet been established whether the Roman and Saxon walls ran altogether on the same line as the later medieval walls, but about 1100 A.D. the city fortifications included the same area as they did to the end of the 18th century.

There were six gates in use in medieval times:-Northgate, Burgate, Newingate, Ridingate, Worthgate and Westgate. Later another came into existence, Wincheap Gate. The walls were frequently rebuilt and reconstructed but never called upon to withstand any real siege after 1011, though the city represented an important strongpoint in the system of national defence.

The West Gate is Grade I listed and a scheduled ancient monument.
The city walls are a scheduled ancient monument.