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The castle stands on Castle Rock. It is the remains of an extinct volcano, which is estimated to have risen some 350 million years ago. The summit of the Castle Rock is 390 ft above sea level, with rocky cliffs to the south, west and north. This means that the only readily accessible route to the castle is from the east, where the ridge slopes more gently, giving the castle its all but impregnable location. It is believed that the first inhabitants of Edinburgh settled on Castle Rock, possibly as far back as the 9th century BC.

There has been a royal castle here since at least the reign of David I in the 12th century, and the site continued to be a royal residence until the Union of the Crowns in 1603 although few of the present buildings pre-date the Lang Siege of the 16th century, when the medieval fortifications were largely destroyed by artillery bombardment. From the later 17th century, the castle became a military base, with a large garrison. Its importance as a historic monument was recognised from the 19th century, and various restoration programmes have been carried out since. Although the garrison left in the 1920s, there is still a, largely ceremonial and administrative, military presence at the castle including a number of regimental museums. It is also famous as the setting of the annual Edinburgh Military Tattoo.

The castle is now in the care of Historic Scotland, and is Scotland's most popular paid-for tourist attraction, with more than a million visitors each year.

Link Edinburgh Castle Official web site
Link comprehensive Wikipedia article

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