Tunnel Passage at Fort Williamsburg is a Grade II* listed building in the Gwynedd local planning authority area, Wales. First listed on 29 May 1968. Tunnel.

Tunnel Passage at Fort Williamsburg

WRENN ID
long-beam-sage
Grade
II*
Local Planning Authority
Gwynedd
Constituency
Dwyfor Meirionnydd MP: Liz Saville Roberts (PC)
Country
Wales
Date first listed
29 May 1968
Type
Tunnel
Address
Tunnel Passage at Fort Williamsburg, Glynllifon, Llandwrog
OS grid reference
SH 460 551
Coordinates
53.07089, -4.29979
Source
Cadw listing

Also on this page: flood risk · radon risk · weather exposure · detailed attributes ↓

Description

The Tunnel Passage at Fort Williamsburg is a significant structure that has been tunnelled through the defensive mound bordering the parade ground. This stone-walled passage descends from the tower and transitions into a tunnel beneath a brick arched opening. Inside the tunnel, the sides are cement rendered, and the pavement features brick tracks.

The fort itself is roughly square in shape, with redans at each of the four corners, surrounded by ditches and revetment walls.

Local climate

Tunnel Passage at Fort Williamsburg sits in one of the wettest parts of the UK, with an average of 1,370 mm of rain a year. This makes it wetter than 97% of listed buildings in the UK. Its exposure to wind-driven rain is greater than that of 98% of listed buildings.

It experiences an average of 20 freeze-thaw cycles a year, fewer than 78% of listed buildings. Of these, 9 follow at least 1 mm of rain in the preceding 48 hours. Average temperatures at this location have risen by 0.3°C per decade since 1991.

Averages for 1991–2020; temperature trend 1991–2025. Figures describe the wider area (a grid cell of roughly 10 km), not the individual site. Contains modified Copernicus Climate Change Service information 2026.

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Nearby listed buildings

  1. Tower at Fort Williamsburg Grade II* 21 m
  2. Magazine at Fort Williamsburg Grade II* 26 m
  3. Gatehouse at Fort Williamsburg Grade II* 29 m
  4. Armoury at Fort Williamsburg including attached screen walls Grade II* 62 m
  5. Barracks at Fort Williamsburg Grade II* 76 m
  6. Outer Defensive Wall to Fort Williamsburg Grade II* 95 m
  7. Cascade at Glynllifon Grade II 237 m
  8. The Mill Folly Grotto including stone bridge, south of the Afon Llifon Grade II 252 m
  9. Fountain at end of east vista Grade II 258 m
  10. Eastern Cavern north of the path near main east vista fountain Grade II 284 m