Chalk is another huge geographical feature when discussing Dover Castle’s attributes. The geology of Dover (chalk) is an easy substance to dig into, making it the site the perfect location to build a system of tunnels. This supports its medieval use of canons and mortars, all the way up too WW2 later on. Source 5 reinforces this idea of Dover Castle being continuously active. It shows Dover Castle’s tunnels acting as a suitable headquarters for ATS Girls 'plotting enemy ship movements' during WW2. Furthermore, making Dover Castle atypical compared to other castles.
Henry II in the 12th Century fortified Dover Castle with inner and outer bailies. Perhaps he saw this location to be a successful future trading route. Or even to act as a frontier Castle/first line defence from neighbouring countries such as France. Its strategic geographical position makes it incredibly hard for enemy ships to land. Source 9 supports this theory. Henry II spent £6440 on building and improving Dover Castle, unlike the £1000-£2000 he spent on 5 other Castles. This source also states Dover Castle acting as a 'Frontier Castle', meaning the first line of defence against European invaders. Both this aspects again, support Dover Castle’s importance and need, compared to other castles.
Dover Castle's port is also a huge geopolitical factor. The port was first developed by the Romans, and maybe why the actual Castle itself was constructed later on. Source 3 links to this idea, as the 15th-17th Centuries show the only repairs on the Castle were carried out before the King's visit, or after an invasion. However, the harbour and the cliffs of the bay were now safeguarded by 'new bulwarks', showing that the harbour could be of some use. Also supporting the idea of trade and communication, as well as its defence, and linking to the idea of factors for change "The offensive potentiality of the Castle"
In conclusion, of all the factors which play a role, the most important one would probably be its proximity to the coast and neighbouring countries. Other factors, such as its chalk geology do support its continual use, and geographical placement. However, I personally think its Dover Castle’s proximity to the coast which is the most important. It feeds all other factors as well, such as trade, defence and travel.