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Rochester Castle has the tallest keep in the country at 113 ft. The Keep was built in 1127.
The castle is a Grade I listed and a Scheduled Monument. It is open to the public and operated by Medway Council for English Heritage.
For a longer description & history see Link

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