This is a huge disappointment after the fun of Explorers Awheel. The children take on exploring Hadrian's Wall, but instead of walking the length of it they stay in a cottage and drive to a different part every day, which really doesn't work to make a coherent narrative. There's a frankly bonkers subplot about an insane man who believes he's one of the last of the Picts, and who abducts another walker. The children (inevitably) rescue the walker, and while the police take the mad Pict away from the top of the mountain where he's been living, they leave the badly shocked and injured abductee to be brought off the mountain by the children and their uncle (why??!!). A real mess of a book.
Originally written in 1939 Explorers On The Wall is the second book in the Explorers series written by Garry Hogg (1902-1976).
It's a fine example of "young adult" literature in its original form, adventure & fun with subtle learning mixed in. In this book a group of four children and their uncle go on a holiday in Britain to explore Hadrian's Wall and the path of the Roman wall which divided the Roman Empire from the Picts.
Little bit of adventure, little bit of mystery, light and enjoyable to read.