What's living alongside the wall
The trail runs through Northumberland National Park for much of its central section — a working landscape, not a zoo — with specific species worth knowing before you walk past them.
The curlew: the park's own emblem, and a conservation concern
The curlew — a large wading bird with a distinctive long, downward-curved bill and an unmistakable bubbling call — is Northumberland National Park's own chosen emblem, and the moorland and rough grassland either side of the central trail sections is real curlew breeding habitat. UK curlew numbers have fallen sharply in recent decades, which is part of why the park treats the bird as a conservation priority rather than just a mascot; walkers are asked to keep dogs on leads and stick to the path across open moorland during the spring nesting season for this reason.
Waxcap grassland: unglamorous, and rare
Old, unimproved grassland along parts of the route — ground that's never been treated with artificial fertiliser — supports waxcap fungi, a group of small, brightly coloured mushrooms that are a recognised indicator of rare, high-quality grassland habitat rather than a common garden find. They fruit in autumn, making a September or October walk a better time to spot them than midsummer.
Rivers you cross
The trail crosses the River Tyne near Wallsend, the River Irthing close to Birdoswald, and the River Eden on the approach into Carlisle — three distinct river systems on one walk, each supporting its own riparian wildlife, from the Tyne's famous salmon-recovery story near Newcastle to the quieter Irthing valley further west.
Livestock, and why gates matter more than they look like they should
Much of the trail crosses working sheep and cattle grazing land. Gates and stiles along the route exist to keep livestock in specific fields; leaving a gate open behind you, even briefly, can let animals into a neighbouring field or onto the trail itself — close every gate you open, even if it looks like it was already ajar.