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The name Saltwood stretches back at least to the year 488 when Aesc the son of Hengist and King of Kent, built a castle on the site. In 833 Saltwood appears on a charter of King Egbert and from that time the names of its owners are, with a few gaps, recorded in ancient documents. The structure was replaced by a 12th century Norman structure, with work extending over the next two centuries. In 1026 a deed signed by Canute among others conveyed Saltwood to the church. From that time on the castle seems to have an uneasy dual occupation between priests and noblemen or garrison commanders. The castle is known as the site where the plot was hatched to assassinate Thomas Becket on December 28th 1170. The large gatehouse was added in 1380 and gun-ports in the gatehouse that date to the period 1385-94 are attributed to Henry Yevele. In 1540 the Castle was conveyed to the Crown but it soon declined into a farm. It became uninhabitable as the result of the earthquake of 6 April 1580 but was restored in the 1880s as a residence once again of the Archbishop of Canterbury. The gatehouse has been used as a residence ever since. The later hall was largely re-worked by Philip Tilden in the 1930s. During World War II, Nazi Hermann Goering ordered the Luftwaffe to not bomb Hythe, as he had designated Saltwood as his post-invasion home. The castle was the childhood home of Bill Deedes and was purchased in 1955 by art historian Lord Clark of Saltwood (1903–1983), then his son Alan Clark (1928–1999), a minister in Margaret Thatcher's government.

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

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