The fortifications on Dover's White Cliffs protected the stretch of English coast closest to continental Europe. The present castle was built in the 1180s, and over the next 800 years its buildings and defences adapted to meet the changing demands of weapons and warfare.

This is one of a pair set on the cliffs either side of the estuary in the 2nd century. Only this, the eastern one, now in the grounds of Dover Castle, survives. The octagonal plan structure, unique in Britain, still stands to 19m, although the upper 6m are a medieval rebuild. It was modelled on the 3rd century BC pharos in Alexandria and would have had a beacon on top. Reused Roman building materials can be seen in the adjacent Saxon church.

St Mary in Castro church, adjacent to the pharos is a heavily restored Saxon structure. The Roman lighthouse became the church bell-tower.

The Avranches Tower is situated in the outer curtain wall om the east and has a deep ditch on one side. The outer bailey (which the outer curtain wall contains) is huge and was completed in the 13th century by Henry III. Dover's walls comprise the earliest example of concentric defences in Western Europe. Carefully planned walls and towers provide overlapping fields of fire.

These can be accessed via the medieval tunnels and lead underneath St John's Tower and provides flanking fire. In 1216, a group of rebel barons invited Louis VIII of France to come and take the English crown. He had some success breaching the walls but was unable ultimately to take the castle. The vulnerable north gate that had been breached in the siege was converted into an underground forward-defence complex. During the siege, the English defenders tunneled outwards and used these to attack the French.

This is the view covering the approach to Constable's Gate. After the siege which highlighted the vulnerability of the north gate new gates were built into the outer curtain wall on the western (Fitzwilliam's Gate) and eastern (Constable's Gate) sides. Constable's Gate is an elaborate entrance, with D-plan towers facilitating flanking fire.

If an enemy breached the outer curtain wall they would then be faced with the intimidating inner curtain wall. The sole gate can only be accessed from this elevated walkway. However, upon penetrating this gate the invader is only taken into an area surrounded by high walls where the next gate that leads into the inner bailey is off set and heavily defended with more towers. The inner keep can be seen soaring above the inner curtain wall.

The gateway just beyond this gate is known as the King's Gateway. Just in shot to the right can be seen a ferry leaving Dover Harbour for Calais.

The keep stands within the inner bailey and this photo is taken from the King's Gate. The inner curtain wall has 14 towers and the keep has walls that are 7m thick in places.

Dover was adapted to meet the new threats that warfare in the 20th century brought and this 40mm Bofors anti-aircraft gun were part of those new defences in 1939-1945.

It's amazing to think that this narrow strip of water has been all that has separated England from invasion over the centuries. Never since William the Conqueror in 1066 has England been successfully invaded and conquered.