There is a fort under this wood, probably from the Iron Age

Where is Bryn Mawr hillfort?

Bryn Mawr rises in the community of Llandysilio, in Powys, less than three kilometres from the border with England. It is a scheduled monument, number MG158: Cadw, the Welsh Government’s historic environment service, lists it as of national importance, which protects it by law.

It is one of a crowd. Within seven kilometres the national list of scheduled monuments holds 5 more hillforts: Llanymynech Hill, 3.3 km to the north-east, Soldier's Mount, 4.0 km to the north-west, the Breiddin, 6.5 km to the south-east, Gaer Fawr, 6.6 km to the south-west and Bryngwyn Wood Camp, 6.9 km to the west. Offa’s Dyke, the early medieval earthwork along the border, passes 1.9 km away, and the remains of a Roman supply depot lie 2.8 km to the north-west.

How can a laser see through a wood?

The aircraft fires laser pulses at the ground and times each echo. Most pulses bounce off the first thing they meet: a tree top, a roof, a hedge. Some pass through gaps in the crowns and bounce off the ground. This survey was flown in winter, on 22 February 2021.

Two maps are made from the echoes. The surface model keeps the tops of whatever the pulses hit, so over Bryn Mawr it shows the wood. The terrain model keeps only the echoes classed as ground and fills the gaps between them. The view at the top moves from one to the other.

The difference between the two is roughly the height of the trees. Inside the ditch of the fort, the tree tops stand more than 9 metres above half the ground and more than 15 metres above a tenth of it; the tallest reaches 22 metres. In the surface model the fort does not show at all.

What is on top of Bryn Mawr?

A single rampart runs round the summit, with a ditch outside it and, beyond the ditch, the trace of a smaller bank. The Royal Commission on the Ancient and Historical Monuments of Wales, which keeps the national record, describes it in its January 2026 entry as a roughly pentagonal fort about 105 metres east to west and 108 metres north to south, enclosing 0.84 hectares to its outer edge. On the north and north-east the rampart runs in straight lengths.

2021 survey, one-metre grid

2006 survey, half-metre grid

Measured from the terrain

• top of the rampart
• bottom of the ditch
• hollow found in both surveys
• hollow found in 2021 only

Read by the national record (Coflein, 2026)

• where the record places up to ten possible house platforms
• gateway and possible barrow

On the north-west side the line of the rampart pulls sharply inward. The record reads this as the gateway, perhaps with the rampart ends overlapping, opening onto a natural gully that climbs to the domed top of the hill, which is otherwise too steep to reach from that side. In the terrain the line of the rampart steps in by about twelve metres at that point.

The fort was known before this survey. An older entry in the same record, taken from an Ordnance Survey card, already describes two lines of scarps round the summit, read as a worn-down rampart and ditch, with a possible entrance on the west. The terrain model shows the whole plan at once, including the ground inside. That older description, about 84 metres north to south by 74 metres, is still the one in the legal schedule of the monument; the 2026 entry measures the fort to its outer edge.

Did people live inside the hillfort?

Inside the rampart the ground is uneven. The record notes up to ten hollows in the southern part that may be house platforms, levelled places where houses stood, and says a finer survey would settle the detail.

A hollow in a terrain model can come from the processing as easily as from the ground, so the same rule was run on two separate surveys: the 2021 one and an older one flown in November and December 2006 on a half-metre grid. The rule counts every closed dip at least 15 centimetres deep and at least four square metres across, more than three metres inside the rampart. Six hollows appear in both surveys, their centres no more than three metres apart; the count changes with the depth threshold, as the notes at the end explain. They are 22 to 52 centimetres deep and 9 to 59 square metres in area. Three lie in the southern half, where the record places its hollows; three more lie in the north.

A hollow is not yet a house. When a tree blows over, its torn-up roots leave a pit, and people also dug small quarries on hilltops for stone. A finer survey would show their shape more clearly; telling a house platform from the rest would likely take an excavation, and the record describes none here.

What is the mound east of the fort?

Some 88 metres east of the fort, in the record’s measurement, a rounded mound stands in an open field, 10 to 15 metres across. In the terrain it rises about two metres above the slope around it, and it is there in both surveys. The record suggests that, unless it is natural, it may be a barrow, a burial mound placed on the approach to the fort. The record gives no date for it.

How old is Bryn Mawr hillfort?

Cadw’s schedule says the fort probably dates to the Iron Age, which in Wales it puts at about 800 BC to AD 74, the year of the Roman conquest. The national record gives no closer date, and it describes no excavation that could provide one.

How many hillforts in Wales are hidden under trees?

The same survey covers the whole country, so the question can be put to every fort. Cadw schedules 410 prehistoric monuments as hillforts or promontory forts; the terrain and surface models cover at least 95% of the area of 409 of them. In 70 of those, about one in six, more than half of the protected area lies under growth taller than five metres. Bryn Mawr is among the 35 where that share passes three quarters: 76% of its scheduled area.

Bryn Mawr is the exception. At 213 of the 409 forts, more than half, less than 5% of the protected area is under trees. The wooded forts are commonest in the south-east: 14 of the 24 in Monmouthshire, Newport and Cardiff, against two of 41 in Gwynedd.

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