Short answers, pulled from the story.
Dover Castle is called the "Key to England" because of its defensive significance throughout history. It sits above the narrowest crossing of the English Channel, making it the primary fortification guarding the most direct route into England from the European continent.
The oldest standing structure at Dover Castle is a Roman lighthouse built in the early 2nd century. It is one of only three surviving Roman-era lighthouses in the world and is also claimed to be Britain's oldest standing building. The five-level, eight-sided tower was built from tufa, Kentish ragstone, and red bricks.
Henry II spent over £6,500 on Dover Castle between 1179 and 1188. This was an enormous sum given that his estimated annual revenue was around £10,000. The great keep, inner bailey, and outer bailey all date from his reign, with Maurice the Engineer responsible for the keep.
Admiral Sir Bertram Ramsay directed Operation Dynamo, the evacuation of French and British soldiers from Dunkirk, from his headquarters in the castle's cliff tunnels in May 1940. A military telephone exchange installed there in 1941 proved so heavily used that a new tunnel had to be dug just to house the power equipment keeping the switchboards running.
A local merchant named Richard Dawkes and just ten men captured Dover Castle in 1642 without firing a shot. Knowing the castle was lightly guarded, they scaled the chalk cliffs at night, attacked the porter's lodge, seized the keys, and entered the castle before the garrison could be mustered.
According to figures from the Association of Leading Visitor Attractions, 368,243 people visited Dover Castle in 2019. English Heritage had spent £2.45 million between 2007 and 2009 recreating the castle's medieval interior ahead of that visitor period.