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Grade I listed buildings and structures are of exceptional, even international importance. There are over 6,000 in the country. Only 2.5% of listed buildings are Grade I listed.
In Scotland the classification is Category A
Index: 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.

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