FORTRESS GLAMORGAN – how the Kingdom of Glamorgan and Gwent was defended against invasion

The main concerns for the Kingdom of Glamorgan and Gwent in the 5th and 6th Centuries were surprise sea-borne attacks to the south coast, invasions through the valleys and attacks across the Severn Estuary.

A sophisticated and well-planned defensive network was created that combined natural, geographical features with forts and earthworks to plug any potentially weak points.  Forts were placed in naturally supporting rings that could be seen from other strong points allowing early warnings of attacks to be passed along and defensive forces mobilised.  The centres of these rings would be the sites for population and learning centres where a central, fortified strongpoint would also be created that would be able to give protection and clear lines of sight for any warning signals coming from the forts forming the rings.

The main invasion routes all involved different types of terrain – cliffs and rocky shorelines to the South west, plains to the east, hills and valleys to the north and north-east and the Severn Estuary to the East.  Rivers played a key role in all these areas and different strategies were used in each are to maximise the advantage that the different terrain types offered.

Perhaps surprisingly, one of the key parts to the defensive system was Cadbury Hill on the other side of the Severn in modern-day Somerset.

The Coastline West of Cardiff

The coastline west of Cardiff is mostly lined with high cliffs and dangerous rocky outcrops with few viable landing points for an army.  There are twelve gaps that create possible landing sites and each is defended by a coastal fortress and or defensive earthworks.

The priorities for the defenders were to spot attacks early, fight off smaller raiding parties before they could do any harm and delay and harass major forces while waiting for defensive forces to be mobilised and brought to bear.

Transporting and landing a large invasion force by sea would have been a very difficult task anyway, especially when combined with the treacherous currents and tides of these waters and the inevitable harassment from the formidable defending fleet.  Key points were also established along outcrops and islands to act as look-out points to give the maximum warning of any attack.

This line of defences against landings stretched from Ogmore fort in the West to Sully Island fort in the East with the defences being in close proximity of each other and mutually supporting.   Evidence for the defences against nine of these possible landing sites can still be seen with the other three having been built over – such as with the power station at the Thaw (“Aberthaw”.)  They are listed below.

1 –Ogmore Fort guarding the River Ogmore

2 – Ewenny Point – earthwork fortification below the later stone castle

3- Dunraven Bay – earthwork fort on cliffs at Trwyn-y-witch

4-  Traeth Bach fort  – earthwork fort above Whitmore Stairs

5-  Nash Point – Castell-y-Dryw – earthwork below village of Macross

6-  St Donats Castle on the cliffs

7- Llantwite Major – Earthwork Castle Ditches just south of Llantwit Major

8- Hafod – Earthworks at Summerhouse point

9- The Thaw.  Site of the power stations and cement factories was once “heavily defended” – from Life of St Illtyd.

10- Porth Kerry – “The bulwarks”

11 – Whitmore Bay – on the East cliffs at Whitmore Bay, Barry Island

12 – Fort on Sully Island

Penarth Head was one of the lookout points and is now, unfortunately, completely built over.  (Penarth is Welsh for “Arthur’s head”)

The Coast and plains East of Cardiff

The coastline east of Cardiff is very different and consists of low mud banks and has no defendable cliffs to draw upon.  There is a vast area of low-lying land protected from the sea tides by the dykes known as the Sea Wall.   The sea wall stretches from the Taff to the Ebbw and Usk rivers and east from Usk to Magor Pill creating a level open area bounded by the rivers Rhumney and Ebbw.

The tidal rises in this part of the coast are some of the highest in the world with the sea level at Barry changing by up to 42 feet twice a day.

This creates severe problems for landing a cohesive amphibious attack and also difficulties for the defenders in knowing where to site any fixed defences as the shoreline varies dramatically throughout the day.

Instead, a strategy was used which relied more on movement and use of the sparse, open terrain and the fact that it was enclosed by major rivers.  A mobile force was created that could attack and harass the invaders in a piecemeal fashion as they struggled out of the mud flats and on to the open plains where they risked being penned in.

Once on to the plains any invading force would be very vulnerable to harassment and hit-and-run attacks by cavalry.  The British army already had a strong cavalry arm and one of the reasons given for the building of the sea wall (attributed to Arthur II’s cousin St Illtyd in the 6th Century AD) was to create additional grazing areas for the growing number of horses without having to affect the existing food production.  This had the added benefit of creating a wide open area where a foot-based army, such as would be landed by raiders, would be at a severe disadvantage.

The use of this area for breeding horses and cattle was still commented on the in the 16th Century with Rice Meyrick describing the area as one vast open cattle and horse grazing country.

With nothing of significance available to an army stuck on the plains the invaders could even be left there struggling for food, water and resupply from the difficult coast.  The would-be invaders would effectively become under siege themselves.

So the challenge for the would-be invaders would be to get off this plain as quickly as possible while fighting off cavalry attacks.  To do this and march on the heart of the Kingdom and targets such as the Royal Centres of Pentre Meurig and Cardiff, the invaders would have to cross the Rhumney river.

Rumney hill has an ancient fort site on one side of the viable crossing point and Penylan was probably fortified on the other side just above Howardian Grammar School.  Unfortunately this site has been built over.  Combined, these forts would also prevent passage up and down the Rhumney river.  The Rhumney also has steep and difficult banks supported by earthworks.  An opposed attack across this river would have been extremely difficult – with or without harassment and threats to the rear while attempting to do so.

The Ely Estuary

Another possible invasion route for the shallow-draft boats of the time would have been to bypass the plains and go straight up the estuary of the river Ely.

To guard against this a line of connecting forts was constructed along the west bank of the Ely with supporting earthworks.  As with the Rhumney. the banks of the Ely river are also naturally very steep and treacherous and mounting an opposed, amphibious attack up these defended banks, across earthworks and avoiding he defensive forts would have been extremely difficult.  Striking east would have been equally difficult and would only have led to the next natural barrier of the River Taff with the first available crossing point blocked by the fort at Castle Mound, Tongwynlais.

These defences of the Ely would also defend against any land attack from the east that had managed to get this far.  Remains of most can still be seen and are listed here.

F1- Careau, West Cardiff –a  Catholic Church can be seen within the concentric circles

F2- just west of St Nicholas – ¾ mile due west of fort on Caerau Hill

F3- 1 ½ miles due west of F2, ½ mile north of Bonvilston

F4- the river swings west and then due north – massive hill fort of Caerau at Llantrisant

These form a ring in the centre of which is the site of an ancient settlement of the period.  Pentre Meyrick at Mynydd Brychan.  Meyrick is thought to refer to King Meuirg – father of Arthur II.  Within this protected ring is also found the sites of the famous monasteries and learning centres of Llancarfan and Llantwit Major.  Also nearby are the sites of Fort Wigau and the royal hunting manor at Miskin – the heart of the Kingdom.

Through the Western Valleys

To the north, the Vale of Glamorgan is protected by a natural ring of steep hills and an invading force would have to move along the valleys following the paths of the rivers.

From the north-west these are the rivers Ely and Taff.  These rivers converge near the strong point of the fort of Caerau at Llanstrisant – the area known as “Arthur’s Butts”.  This was a key point to protect against not only Saxon invasions but any attempts by other Welsh kingdoms to the north and west.

The massive fort strongpoint of Careau at Llantrisant effectively blocks all entry to the western valleys

The route of the Taff was further guarded at Castle Mound as already mentioned.  There are more forts and earthworks further up the valleys towards Brecon.  These also form defensive rings to protect population centres and will be covered in more detail in a future article.

Aberdare, for example, was described as recently as the 16th Century as still being a horse breeding area.   John Leland wrote in the 16th Century  “Where is as in the Lordship of Mischen in the paroch of Aberdayer a great Race and Bredth of horses 8 miles”.

Through the Eastern valleys

Over to the east a curtain of fortifications blocked the entry to the plains east of the Usk and lined the hills.  Down on the plains stood the stone walls of Caerwent and Cardiff.

The forts protecting from attack from the east of the Taff are –

1- Tongwynlais – to protect the gateway to the Valleys.  Supported by earthworks-

1a- one mile west at Tyllo Morris

1b- one mile west on the Wenallt above Rhiwbina.

2- Rhiwbina Hill above Cardiff

3- Above the Dyffryn plains – 1 fort either side of the hills above the pass where the Ebbw river runs down from the valley.

4- Caerleon protects the pass of the River Usk .

Earthworks run along the hills above Llandebr and Penlaw

5- Caerwent fortress blocks the road from the east

6- Caldicot supporting Caerwent .  King Meurig is said to have strengthened Caldicot Castle which is more evidence of the 6th Century system.

7- Earth work fort at Wilcrick – 3 miles west of Caerwent at Magor.

8- Pass of the River Ebbw at Tredegar Park, Newport – 2 forts

9- Entry to Machen Valley past Basseleg on the hill west of Rogerstone – supporting 8

Attacks across the Severn and the significance of Cadbury Hill

The most significant threat to the Kingdom in this era was from the growing strength of the Saxons and populations of southern England that had been absorbed by them.  The major barrier to this threat was the formidable Severn Estuary which boasts the world’s second highest tidal rise and fall.  Until the building of the Severn crossings in modern times the only way to avoid this crossing was by taking a long detour north to where the river is smaller and then make the long journey back down through the valleys as described above.

If crossings were attempted further north the natural barriers of rivers and forts at places like Caerwent, Caldicot, Caerleon and Wilcrick would come into play and make progress very difficult.

Any would-be invaders would still have to cross the defences described above before crossing the plains of Cardiff and on to the fortifications and rivers there.

The most direct and quickest route to the Glamorgan and Gwent heartland would be to avoid all of those obstacles and cross further south directly to the more open shoreline and plains.

The river narrows opposite Caldicot and a crossing could be attempted.  This would still be treacherous due to the tides and defending fleet but if surprise could be achieved it was the most direct way to bring overwhelming force to attack South Wales.

This is why Cadbury Hill, while not in Wales, was so crucial to the defensive system.

As witnessed by recent archaeological activity at Cadbury hill, this large, 18 acre Iron-age site was reactivated and rebuilt in this period.

Its function was to deny the south coast of the Severn to would-be invaders and give warning to the garrisons on the Welsh side of the Severn to prepare.  Any would-be invader would also be at risk if Cadbury was left in their rear when attempting any crossing.  Attacking Cadbury Hill would be a large and time-consuming endeavour removing all elements of surprise and shortening the campaign time that would be available.

The fort is beautifully positioned to be seen clearly from all the major fortresses on the northern, Welsh side of the Servern – Cardiff Castle, the Caerau Llantrisant hill fort, Penarth Headland and the forts around Newport and Usk all had uninterrupted views and would be able to see any messages signalled from Cadbury Hill.

Cadbury Hill could also be supported by use of the fleet operating from the Welsh side of the Bristol Channel.

I am totally indebted to Wilson and Blackett for their research into Fortress Glamorgan and them kindly giving permission for their results and observations to be used in this article.  As they say themselves –

‘Whether this fortification system was built in the 4th or 6th Century is not the point.  It could have been reactivated for use.  The evidence from Dinas Powys and from Cadbury Hill is that these hill forts were in use in the Arthurian period.’

The plan is to visit all of these sites and carry out further research.  Group visits are being planned and if you would like to join us for some days out or join the discussion then please sign up for the newsletter, join the Facebook group or follow these pages for more information.

Incidentally, the invasion route that the Saxons finally chose was via Gloucester and this ambitious attempt culminating in the battle of Badon is the topic of a forthcoming article.

The original Wilson and Blackett research and writing on this can be found in “Arthur King of Glamorgan and Gwent” Pages 156-161

4 thoughts on “FORTRESS GLAMORGAN – how the Kingdom of Glamorgan and Gwent was defended against invasion”

  1. Thanks for this, it makes it very clear how this whole area was a grand strategic military complex and, I suppose, chosen for this reason. This makes it very clear how formidable the defences were of Arthur’s kingdom. Thank you for setting it out so clearly. I’ve made a copy so if I get the chance to holiday again in the Cardiff area I will have a plan of all the places I want to visit. The British Khumry were master military strategists and they had the art to perfection. Most scholars seem to attribute the more obvious forts to Roman design which doesn’t seem to match this evidence. Romans seem to have preferred walls and enclosures as at Hadrian’s Wall country.

    1. Quick update – along with some friends who I made through this project we have now made videos on some of the sites mentioned and they are being posted on the ‘BritainsHiddenHistory Ross’ channel.
      A new forum has also been set up at where this and other subjects are being discussed.
      Sorry for the inconvenience in moving the forums. It allows other moderators and admins to be involved more easily and hopefully will be more accessible for more people to be involved – which is the whole idea of doing this! Hope to see you commenting there and maybe in the rapidly growing britainshiddenhistory facebook too.
      The ‘BHH forum’ link at the top of this website’s homepage will take you straight to the new forums.

  2. Thanks for the comment.
    There are going to be some group visits to sites in South Wales this summer that we are organising through the Facebook group. I will also try to get dates on here as well when the details are confirmed

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