The west end of an unmapped tunnel exists here. See 2024-12-31 photo:
https://postimg.cc/dLgdyXy5

The right of way is broken due to a small landslip that wiped out a set of steps. See 2024-12-31 photos.
https://postimg.cc/6y8bKkKc
https://postimg.cc/1fd0KXLc
It looks like 30-40 steps in total, with about 5 missing.

The footway disappears here, at the edge of a large landslip.

The Northumberland County Definitive map (online) has not been updated.
(See https://northumberland.maps.arcgis.com/apps/webappviewer/index.html?id=59bccc5416b144a28def537994bf7d10, route 513/030)

Photos of the landslip :
https://postimg.cc/gallery/PkMM3Hrc
and a printed closure statement:
https://postimg.cc/TLnNzgX6.

Note that there is an unmapped railway tunnel nearby. That will be reported separately.

Bridge at 54.63918, -1.378848.

Photos: https://postimg.cc/gallery/KhjjWyy9

First photo looking south.
2nd photo looking west, on the bridge
3rd photo looking east along foot track.

Reference point. Brierly Drive between this point and Castle Eden Walkway is mostly nonexistent.

This segment of Brierley Drive (between Castle Eden Walkway and a point hundreds of metres further east) exists only as a faint contour in a thicket.
There is no navigable path.
The route does pick up again, but I have not mapped that point yet.

Some nearby images for context.
https://postimg.cc/gallery/T6xw7vP4

Traces of the drive contour are evident at the woodland edge, but parts have been obliterated by recent landscaping.
https://postimg.cc/v4XZCGcL
https://postimg.cc/RN7FQ7mY

Culvert exists here. As does Brierly Beck.
See 1897 OS map for reference.
https://postimg.cc/gallery/FX7zBxV0

Part of this route follows the beck, east and west of the walkway, and part follows the traces of Brierly Drive.
https://postimg.cc/9D5QPX8N

Gates here. One swinging wooden gate for pedestrians. Forgot the look at the wider one next to it.

On Enviroment Agency maps, this watercourse is called Twizell Burn, connects via pipe to the outfall at Tyne Road East.