Portland Roads is one of the best natural harbours on the south coast of England, being sheltered by the mainland on the north side and by the Isle of Portland on the west and south sides. This made it an obvious site for a landing, offering shelter for an enemy fleet whilst troops could be landed on Chesil Beach (between Portland and the mainland) or near Weymouth.
In 1539 King Henry VIII'of England, fearing an invasion from France and Spain, instigated a programme of coastal defence. As result a large number of forts, known as the Device Forts, protecting key harbours and vulnerable landing sites were built along the south coast. The Portland anchorage was identified early on as being vulnerable.
In the 16th century the effective range of artillery was not great enough to cover the whole anchorage from a single point, so two forts were built. In the south was Portland Castle and in the north was the slightly smaller Sandsfoot Castle. There were two smaller works at the town of Weymouth, a short distance east of Sandsfoot. These consisted of a battery on the Nothe Point and a blockhouse north of the river mouth. They were probably earthworks mounting a few guns each, designed to guard against an attack on Weymouth.
Portland Castle was built on the shore so that the guns, which were most accurate when firing on a level trajectory, could hit enemy ships on the waterline. The fort consists of a curved battery facing out over the harbour with a circular keep behind.
The landward side is formed by the rear of the keep and two wings radiating out from the keep to meet the outer edges of the curved battery. The curved battery, which provided most of the coastal firepower, had two storeys.
The lower storey had 5 guns, each with its own arched embrasure'and smoke vent to prevent the build-up of powder smoke as the guns fired. This was covered with a stout timber roof supporting the guns above. On the upper storey there were 4 guns in the open air.
The guns on the upper storey were positioned above the pillars between the arches of the lower storey for support. Behind the curved battery was a third tier of guns on the roof of the keep and its wings.
On the roof of the keep two guns could be mounted and the east and west wings each mounted a single gun. In addition to these 13 guns inside the castle, there were plaftorms for extra guns on the west (mounting a single gun) and east (mounting 3 guns) sides of the castle.
The landward defences were minimal, with no flanking capacity to the rear of the keep and its wings. There were archaic cross loops for handguns all over the rear walls of the castle and the guns on the roof could possibly have been turned round to face the land if necessary.
The castle was constructed in a relatively short period, being essentially complete by the end of 1540 just over a year after it was begun. At some point, possibly in 1545, a moated courtyard was added to the rear of the castle, providing more space for outbuildings.
On the north side of the anchorage, Sandsfoot Castle was vastly different in situation and design. The main reason for this difference is probably the geographical differences between the two locations, which made it impossible to build Sandsfoot at sea level, so its gunnery would be less effective.
At the same time, the cliffs on the north side of the anchorage offered no suitable landing places. So unlike Portland, the main role of Sandsfoot was not to defend against a landing, but to prevent enemy ships taking refuge to the north out of range of Portland's guns.
Sandsfoot Castle consisted of an octagonal gun tower with a rectangular accommodation building extending inland behind it. This design was also used for the Device Forts of Brownsea Blockhouse (protecting Poole harbour) and St. Andrews Blockhouse (protecting Southampton Water).
It seems that initially little thought was given to landward defence and the castle was a coastal battery with quarters for the garrison. Although there may have been gun-loops on the ground floor, the windows on the second storey were large and vulnerable.
Between them, Portland and Sandsfoot Castle covered most of the anchorage with their guns, preventing an enemy fleet from using it to assemble in safety. However they were neglected for much of the 16th century and by the time of the Spanish Armada in 1588 coastal erosion was already taking its toll on Sandsfoot, which was in danger of subsiding and Portland was in poor condition.
Before the Spanish Armada, the defences were hastily brought up to scratch and they came close to being put the test when the Armada met the English fleet in battle just off Portland Bill. In the end however, the Spanish continued east to the Isle of Wight, before heading for Calais.
In the ensuing years, the castles fell into disrepair once again until the 1620s when they were surveyed and it was decided to strengthen the now obselete forts.
In 1623 an earthwork trace with two bastions'on the landward side was constructed around Sandsfoot Castle. Probably at the same time, Portland Castle was strengthened by the construction of an earthwork bastioned trace on the landward side.
These earthworks allowed the 16th century coastal forts to continue to function but provided them with much-needed protection against an assault from the land.
Both castles were put to test during the English Civil War, when Portland Castle was initially Paliamentarians, but was captured in 1643 when Royalist troops disguised as fleeing Parliamentarian soldiers gained entry. In 1644 the Parliamentarians returned, taking Sandsfoot after a short siege.
Portland Castle however, proved much tougher and held out for 4 months until a Royalist relief force arrived. A further attempt on the castle in 1645 also failed and it remained in Royalist hands until the end of the war.
This success may have been largely due to the high morale of the garrison, which was mostly composed of local men who who determined to defend their local area. That said, the earthworks constructed in the 1620s made Portland Castle one of the few places in England to have modern fortifications by the time of the Civil War, which must have been a key factor in enabling the garrison to hold out so successfully.
After the Civil War Sandsfoot was considered too damaged by erosion to be of much use and as the range of artillery increased it was evident that Portland Castle was sufficient to provide a degree of protection to the whole anchorage.
The next major change came in the mid 19th century when work started on building a huge breakwater to enclose the Portland anchorage, forming Portland Harbour. New fortifications were also built to protect this new harbour.
In the 1850s a large fort was built on Verne hill, behind Portland Castle. This fort, called the Verne Citadel, overlooked the whole anchorage and its guns covered the approaches.
In 1872 another fort was built on the site of the Henrican battery at the Nothe point. The Nothe Fort and the Verne Citadel were armed with long range rifled artillery to combat the advances in naval firepower and ensure the protection of the harbour.
Portland Castle, the Verne Citadel and the Nothe Fort were all still in military hands during the Second World War. Portland Castle was soon decommissioned, the Nothe Fort lasted until 1961 and the Verne is still used as a prison.
Portland Castle now belongs to English Heritage and is open to the public daily between April and October. It has been restored to be as close as possible to its 16th century state, although the floor of the second storey gun platform is still missing.
Most of the castle is accessible and there is an audio guide explaining the its history and purpose. Sandsfoot Castle has been badly damaged by weathering and erosion, so that most of the gun tower has fallen away into the sea.
The rectangular accommodation block survives but is surrounded by railings to prevent access (for safety reasons). The early 17th century earthworks around the castle have also mostly survived, although they have slumped from their original height over the years.
Sandsfoot Castle stands in an open park and can be visited free of charge. It is possible to walk between it and Portland Castle via the old Portland railway path and then along the spit to Portland.
The Verne Citadel is still used a prison, originally set up in the 19th century for the convicts used to build the breakwater. It is not possible to visit the inside, but a short walk up the hill from Portland Castle will bring you to the northern entrance.
There are footpaths leading around the outside of the citadel, so it is possible to see most of its defences, but only from the outside.
The Nothe Fort, which houses the Museum of Coastal Defence, is open to visitors and is situated in Weymouth, where there is a train station. It is possible to visit all the various fortifications in a day walking along the South West Coast Path, which passes close to all four.
The walk from the Nothe to the Verne via Sandsfoot and Portland Castles is roughly 5 miles in length. The nearest station is at Weymouth, about 5 minutes' walk from the Nothe Fort.