Dover Castle, a large coastal fortress is often called the "Key to England" due to its strategic defensive position. This large and beautiful castle has played major roles in wars through the ages including housing the naval command centre in its secret tunnels during the Second World War.
What I love about this 11th century castle (in addition to it being a day trip from London) is that the structure is intact and some of the rooms are staged to help you imagine how they were furnished during medieval times when it was first built.
While exploring the castle grounds, make sure to also visit St. Mary in Castro sitting on the White Cliffs of Dover. This church fell into disrepair for centuries and was restored in the mid-1800s and is still an active place of worship.
While visiting Dover Castle, it's also worthwhile to take a stroll along the well-marked trail along the top of the White Cliffs of Dover. You may be wondering...what makes the cliffs so white? The cliffs are actually composed of chalk! The layers of chalk or very fine limestone are 300-400m deep as a result of millions of years of build-up of the skeletons of planktonic green algae. How cool is that?
From here you can head into town for a wander or head north to visit Henry VIII's elaborate chain of coastal fortresses including Deal and Walmer Castles built to defend the country against a French attack. Walmer was one of my favourites because it also had beautiful gardens.
Walmer Castle top and garden right, Deal Castle left
All the castles are owned by English Heritage so it may be worthwhile to buy an annual membership for £54 since just visiting the castles in this post add up to £36.70.
£19.40 Dover Castle
£10.70 Walmer Castle
£6.60 Deal Castle
English Heritage also owns Stonehenge (£15.30 admission) if that's on your sightseeing list.