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Barnard Castle Walls Achilles heel and missing bastion

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Sitting high upon its sandstone bluff overlooking the sheer cliffs of the River Tees  – the question that has been asked is – did the indomitable medieval Norman fortress of Barnard Castle and bane of the Scots have one great geological weakness and did the castles medieval master builders know this?

Was the Achilles heel of the castle the presence of shale bedding layers and particularly one unexpectedly very thick layer in amongst the sandstone upon which the high foreboding and impregnable castle walls were built?


Barnard Castles high north walls – sandstone with thin shale beds

Over millennia of exposure to the weather the shale in amongst the stronger sandstone supporting the foundations of the castle walls has slowly become weaker, softer and more easily eroded allowing the sandstone blocks upon which the castles foundations stand to loosen, topple and slip with time.


Soft black shale at Barnard Castle

One very old historical eyewitness account describes a sudden large collapse of the castle walls during the night along Bridgegate perhaps attributable to a slip of the weak shale rocks upon which the walls are in part founded in this area.

Though the shale is typically thin and less than < 0.5m an unexpectedly larger layer of some 8m thickness rises southwards from Castle Bridge up the steep west facing castle slopes along the A 67 Bridgegate towards the town below what was once the medieval outer ward of the castle.

Today though this thick dark shale band is mostly hidden by dense wood and ivy overgrowth it can be seen in an exposure of rock which the locals call the “Crows Nest”. It is warned however that this location should not be visited as the overhanging rock here is potentially unstable and dangerous. In addition the steeps slopes on which this feature is to be found have recently been fenced along Bridgegate for public safety.

“Crows Nest shale” – clearly highlighting the risk of tunnelling and undermining of the shale and the castles west wall above it

“Crows Nest shale” – clearly highlighting the risk of tunnelling and undermining of the shale and the castles west wall above it

It is the shale rock and this thick unit in particular at the Crows Nest which was possibly the greatest natural weakness of Barnard Castles defences in medieval times though today the shale both here along Bridgegate and elsewhere still continues to pose problems for the stability of the castle slopes and walls.

In medieval times the main concern for the Lord of a castle would have been siege warfare and the breaching of the castles defences by tunnelling or undermining, the purpose of the later to bring about a catastrophic collapse of the walls through which an  enemy army – in this case usually the Scots could surge. On several occasions in the 13th Century the Scots laid siege to Barnard Castle though it never fell to them. In one battle in 1216 King Alexander’s brother-in-law, Eustace de Vesci, was killed by an English crossbow bolt fired from the castle walls. The castle survived that siege, but in 1264 was taken by barons supporting Simon de Montfort’s rebellion against Henry III. Subsequently, the castle came under the ownership of Richard the III and was one of his favourite northern strongholds.

On the east side of the castle next to the old town the shale and the security risk posed by tunnelling through it was dealt with by the construction of a massive flooded moat or ditch perhaps up to 5m deep. This defensive structure provided a watery cut-off and deadly trap for miners effectively extending the castles defences downwards to a considerable depth into the ground. Evidence for this moat was established by Geoinvestigate several years ago in Boreholes sunk just outside the castles old walls. These Boreholes encountered thick organic debris infilling the moat underlain by weak shale beds which had been excavated to form the moat. Such weak rock could easily be dug by medieval tunnellers or today’s equivalent – military engineers or sappers. The term sappers comes from the French “saper”, to dig or to trench. Similar weak shale beds can be seen exposed in the banks of the inner moat surrounding the main keep and the massive fortification of Constable Tower.

Medieval sappers

Medieval sappers

On the castles steep west facing slope the shale and the security of the west wall was considerably more problematic and serious. Though the natural slope upon which the west wall was located is high and steep and seemingly offers a formidable obstruction to scaling there is one very major security problem. Here the shale is much wider being 8m thick. Exposed as it was part-way up the steep cliffs supporting the high castle walls it wasn’t possible to dig a moat through it. If an attacking or besieging army found the shale they could quickly tunnel deep into this weak layer entering somewhere far within the castles defences with deadly surprise. Alternatively they could cause a major breach of the defences by undermining and collapsing a section of outer wall possibly followed by the inner wall as occurred at Rochester Castle in 1215.

The siege at Rochester gives an idea of how medieval sieges were conducted and might have been prosecuted at Barnard Castle. Initially King John’s army fired missiles and crossbow bolts into Rochester Castle’s defences and hurled heavier projectiles using several large siege engines. These however proved ineffective and John turned to other methods to breach the defences. A letter dated 14 October indicates John was preparing to undermine the castle’s walls. He wrote to Canterbury, asking for the production “by day and night of as many picks as you are able” and that they be sent to Rochester. On 26 October a relief force of 700 horse was sent from London. They turned back before arriving, perhaps because they heard the king was advancing to meet them.

Medieval castle siege

Medieval castle siege

A letter sent from Rochester on 25 November offers insight into the methods of medieval siege craft. John ordered Hugh de Burgh to “send to us with all speed by day and night forty of the fattest pigs of the sort least good for eating to bring fire beneath the tower”. The wooden props supporting the tunnel dug beneath the keep were set alight to collapse the mine, bringing down one corner of the keep. Still the garrison held out and sought safety behind the stone partition or cross-wall in the keep, abandoning half the building.

Eventually the inner keep fell and the defenders surrendered. Initially John wanted to execute them all as was the custom of the time when a garrison had forced a long and bloody conflict. Savaric de Mauléon, one of John’s captains, persuaded the king otherwise, concerned that similar treatment would be shown to royal garrisons by the rebels. Only one person was executed: a crossbowman who had previously been in the service of the king since childhood was hanged.

Mining was undertaken at Rochester Castle in 1215, at Dover Castle in 1216, and at Dryslwyn Castle in 1287.Uniquely at Dover Castle the English defenders tunnelled out to attack the French besiegers and created a counter tunnel which can still be seen in the medieval works. The practice has left us reminders in English. Undermining has acquired figurative as well as literal meaning and military engineers are still known as Sappers.

So getting a castles defences right and identifying and counteracting potential weaknesses particularly those which could facilitate tunnelling was a matter of life or death to the medieval master castle builder. Tunnelling was a very real threat and undermining of a castle wall or tower was one of the most feared events by the Lord of the castle, for it would surely lead to his defeat and the massacre of all inside. To defend themselves from tunnelling castle dwellers put out a bowl of water and watched for ripples that might indicate digging.

The oldest known sources of employing tunnelling and mining to capture enemy defences are Roman and its ramifications were well known to medieval castle builders and to the builders of Barnard Castle which was a major front line fortification at high risk of attack from the Scots who were a very serious threat for much of the 12th and 13th Centuries.

At Barnard Castle the section of west wall underlain by thicker shale being fairly straight in line and standing as it does on a steep slope would have created a blind spot where any activity on the slopes below the wall and the shale could not be easily seen or repelled from above or from the side. Because of this blind spot archers or bowmen on the castle walls positioned above the slope or some distance further along the parapet wall could not get a good shot at an enemy army that might be trying to tunnel their way through the shale rather than trying to scale the murderous steep west slopes.

How then could this potentially catastrophic weakness on the castles west slope be countered or repelled? One suggestion made by Geoinvestigate during their investigation of the stability of the castle walls was that a “bullnose” or a major semi-circular bastion of some 25m length and 12m radius might have been constructed at the south end of Bridgegate Road next to the existing block of flats at this location. The bastion extended out from the line of the current wall and sandstone cliff towards the nearby old medieval road and the River Tees. On further investigation it is now considered possible that the bullnose may have been 40m or so in length depending on where its northern return was located which is uncertain because of the alteration of the walls in this area.

Looking north from a high vantage point on the bastion walls there would now be no blind spot and the entire west slope and its weak mineable thick shale layer could be swept by deadly crossbow fire in a narrow killing zone over a distance of some 100metres between the bastion, the bridge and the river. Other possible advantages of locating a bastion at this location would be the command it offered of the road and the main bridge as well as perhaps the control an adjacent fording point on the River Tees downstream of the bridge.

The crossbow played an important role in the late Medieval period. The crossbow was really the first hand-held weapon that could be used by an untrained soldier to injure or kill a knight in plate armour. The most powerful crossbows could penetrate armour and kill at 200 yards or 180 metres. Longbowmen could certainly penetrate plate mail (though perhaps not at such a great distance), but longbowman were generally highly trained soldiers. This meant that they were also expensive, and that they could not be replaced easily. Many bowmen were recruited at a young age to master their craft. The advantage of the English longbow however is firing rate and distance and in skilled hands it could fire 2.5 times faster than a crossbow and attain distances of 250 to 350 yards.

Though the bastion structure at Barnard Castle cannot be seen today and its existence is still uncertain and a matter of conjecture there are several pieces of information which (apart from the defensive measures that must surely have been implemented by the castle builders to counteract the very serious weakness posed by the shale layer from tunnelling) may support the argument that this structure was once there.

The pieces of information include a Georgian map showing a double curved castle wall or “bullnose” feature next to the flats, an early blurred photograph which may appear to show a partially broken down and dilapidated circular wall in this area and an early Ordnance Survey map showing a semi-circular land use pattern broadly corresponding with the photograph and the Georgian map. In addition there are several deep cuts or sockets in the rock faces at this location of which one is particularly curious being particularly large and deep.

Georgian map c 1770s showing bullnose or bastion lower right

Georgian map c 1770s showing bullnose or bastion lower right

The Georgian map shows a moat was present on the south side of the bastion in the area now occupied by the flats and that a narrow bank or pathway ran around the bastion and up the west slope to the base of the castles west and south walls from this location.

Enlarged Georgian map top 19th Bottom Late 19th C Ordnance Survey Plan. Note smaller triangular area of white surrounded by black below the figure 6

Enlarged Georgian map top 19th Bottom Late 19th C Ordnance Survey Plan. Note smaller triangular area of white surrounded by black below the figure 6

Interestingly the Georgian map suggests that the bastion may have been asymmetrical with its tighter curved section overlooking the west slope and the rising thick shale layer – perhaps in this way maximising fire power from longbow and/or crossbow both parallel to the line of the slope and onto any location on the slope face itself. A modern day comparison would be with that of machine gun fire from a pillbox or machine gun nest. As such during a siege it would suicidal and nigh on impossible to gain a foothold on the west slope to commence tunnelling or for that matter to scale the slope and attack the walls with ladders.

Late 19 th C photograph showing what might be the dilapidated broken down curved bastion wall and grassy mound below trees and above roofs. River Tees at bottom. North to left south to right.

Late 19th C photograph showing what might be the dilapidated broken down curved bastion wall and grassy mound below trees and above roofs. River Tees at bottom. North to left south to right.

Both the late 19th C photograph and the OS map of the same period show good similarity. Looking at the photograph a tree appears to stand on the top of the grassy bastion mound. One particularly curious feature – a rectangular tower like structure also located on the mound to the right of the tree also appears on the OS map   below the figure 6. A possible interpretation for this is a stairway up to the bastion platform which originally occupied a higher level but which has subsequently eroded and tumbled down as a result of the removal of the upper section of the outer wall.

Similar medieval bastions can be seen today on the outer and inner walls of Caerphilly Castle overlooking and protecting the main outer moated drawbridge and the inner drawbridge and it was not untypical for medieval master builders to move from castle to castle building works applying or adapting castle designs they had used before.

Caerphilly Castle showing semi-circular bastions

Caerphilly Castle showing semi-circular bastions

There are 5 major semi-circular bastions at Caerphilly the largest on the left of the inner drawbridge having similar dimensions to Barnard Castle of 40m length x 12m width.


Caerphilly Castle plan – scale bottom left

Caerphilly Castle plan – scale bottom left

Perhaps the most convincing clue to the presence of the bastion that can be seen today at Barnard Castle is a massive deep groove or socket hewn into the sandstone rock face next to the block of flats on Bridgegate.

The obvious explanation for this feature is that at some earlier time a substantially thick robust wall was once tied in to it though no trace of this wall remains today. Interestingly as would be expected the trajectory of this socket which lies at the south end of the bastion corresponds with the line of the bullnose wall shown on the early Georgian map.

South bastion socket at chainage 130m  – V cut  in rock points upwards to top right corner of photo

South bastion socket at chainage 130m – V cut in rock points upwards to top right corner of photo

South socket next to block of flats. Fence line shows alignment of socket

South socket next to block of flats. Fence line shows alignment of socket

The position of the northern re-entry of the bastion wall on the west slope is less clear hence the uncertainty over its length. However there is an unusual feature at chainage 90m on the sandstone rock face which at the position where the late 19th C Ordnance Survey map suggests that the wall may have joined the main west wall. Initially Geoinvestigate thought this might be a tunnel which was subsequently infilled and sealed.

Deep cut in sandstone face at chainage 90m. Possible bastion north socket

Deep cut in sandstone face at chainage 90m. Possible bastion north socket

An alternative interpretation of the 19thC map is that the bastion was smaller being 25m in length rather than 40m. Once again examination of the cliff face shows a sealed vertical cut in the rock face at chainage 105m where the alternative position for the northern re-entry of the shorter wall would be expected to occur.


Two possible interpretations of bastion size with northern sockets at ch 90m and ch 106m


Possible alternative position of northern socket at ch 106m


Perhaps the bastion was constructed at a later date as an add on when the castle defences were extended outwards or perhaps after the shale had been discovered below the castles wall or when the castle defences were upgraded and strengthened from time to time as was done in the late 13th Century. Or perhaps this structure was added soon after the sieges of 1215 and 1216 at Rochester and Dover Castles which would have been sharp reminders of the vulnerability of even the greatest of castles to tunnelling.

A highly tentative impression of what Barnard Castle missing bastion at the south end of Bridgegate may once have looked like has been provided for the curiosity of our readers. Perhaps, sometime in the near future once the evidence has been reviewed we may have an answer as to whether the greatest weakness to Barnard Castle’s defences was a natural geological defect (perhaps unrecognised or overlooked when the castle was first built or extended) rather than the design of its massive walls and defences.

Barnard Castle

Or perhaps this geological weakness was never identified by the castles master builders or was it kept secret by them and the semi-circular bastion never existed? But what then is the explanation of the bullnose feature shown on the Georgian map and the massive deep rock cut on Bridgegate next to the flats ? Furthermore what counter measures would the medieval builders have taken to protect the castle against the breaching of its west defences through the Crows Nest shale on the reasonable assumption that these master builders and their client lords were learned in the medieval siege craft and it is unlikely that they would not have spotted a major geological defect such as the Crows Nest shale.

Related Links:
Engineers report on the stability of Barnard Castles, Castle Walls – Durham County Council – 2014
Flicker image of Barnard Castle.
Engineers Report on the Stability of Castle Walls