A parliamentary fort erected to defend London during the Civil War.
The picture source website is fascinating but strangely we can't actually locate Stones End on the maps there. There used to be a concrete map, 'Lines of Communication', in Bishop’s Square, Spitalfields which showed the Civil War fortifications around London and that didn't show Stones End either. 2015: Matt at Londonist has walked the northern section of the Civil War defences. 2017: IanVisits has a good post and is doing the whole circuit. 2019: Spitalfields Life has photographed lots of the sites.
There were 24 forts with trenches and ramparts between them all constructed 1642-3. Dutch engineers advised on the fortifications and a sixth of the population of London were involved in the works. Information from 'Samuel Pepys' by Claire Tomalin.