The site of which Dover Castle is built upon, may have been founded and slightly fortified in the Iron ages, even before the Romans first invaded Britain in AD43. However it wasn't until Henry II, in the 12th century first took an interest to Dover Castle's geographical location, that it could be recognised as a potential frontier Castle.
Dover Castle is often described as the 'key to England', which in my opinion highlights Dover’s strategic geopolitical importance. This Castle is the first line defence and entrance to London, by both sea and land. And shows some perspective of how important Dover Castle must have been. This Castle was also vital for protecting the capital of England, heart of the British Empire. Source 2 also briefly states Dover Castle being seen from London roads as 'Majestic', and a 'Fortified City' rather than just a Castle. This is somewhat a symbolic and geographical factor. (The Castle was almost indestructible as it was so high up, but also signified Britain’s power.)