Heralded as "The Largest Castle in England", Dover Castle was another possible Roman fortification (43 A.D.). The standing legend regarding Dover's construction centers around William The Conqueror. On his march to Westminster Abbey (to be crowned as King of England), he passed the steep cliffs of Dover. Noting the need for a fortification to defend the area from future invasions - he ordered a stronghold built. Dover turned out to be the "first" of the traditional English castles constructed under orders of William. Documents show that it was first garrisoned by 8 Knights as early as 1088. Although no where near the original design of today, it was more than likely a Motte and Bailey classification. Utilizing the existing hillsides andf natural defenses - perimeter walls were constructed to run into many of these hills (to enhance their defenses).
By the reign of King Henry II, Dover Castle began to take on it's now traditional form. Henry spend large amounts of funds to convert and enhance the fortification with stone work. Dover was first "tested" in 1216 during The Baron's War. A French invasion force attempted to storm Dover Castle. They were unable to breach the inner Baliey. Heavy damage was down to it's Gatehouse and walls. When rebuilt, a new design involving underground construction was applied. Much of this new construction was begun under Henry VIII. Dover was occupied by Royalist forces during The English Civil War (Parliamentarian forces still were able to take if without firing a shot).
Due to this, it was never rendered "slighted" (as was the fashion). During The Napoleonic Wars of the 18th/19th Century, many military engineering renovations took place (buildup of heavy coastal artillery emplacements). During World War II, some 2,000 soldiers were garrisoned there (again fearing invasion). Most of these troops spent a vast majority of their time underground (a first). Also during the World War, the evacuation of Dunkirk was planned at Dover. Dover is a massive and well spread out structure that is open for Public viewing.