@Infoseepage There is a little one just up the road from me (Washing Wells), an early fort which is only about 150m square. Nothing left visible from the ground there now.
@Infoseepage Hmm. Yes there does appear to be a circular palimpsest of some sort. When I was a very young lad I remember it being surveyed from the air. Walked over it countless times but it just looks like any other farm field really. There was a colliery nearby up until the 1960s so there may be other reasons for surface disturbance.
@Infoseepage Had a look for it on the wonderful NLS georeferenced maps. There was still nothing there on the 1850s OS map, but their satellite overlay layer shows the circular pattern just as clearly.
@Infoseepage The FB post led me to look for the Arbeia Journal. There is an article on the fort in Journal 3 (see link for pdf) which mentions a previous excavation adjacent the site which unsurprisingly found old mine workings (no surprise, they are found pretty much everywhere around here). Apparently there is another article in Journal 10 which I have yet to obtain. https://www.arbsoc.org.uk/arbeiajournal