These ruins are the remains of Gunpowder mills that once operated within the wood, powered as they were by water from the river that flows through the place. Near the the Staines road entrance there is actually an old millstone lying beside the path. Another feature of Donkey Wood are the hillocks and earthworks to be found within it. The hillocks are man made and called 'Baffle Mounds' as they were constructed to baffle explosions that I presume would occur unexpectedly from time to time as a result of messing about with gunpowder? I guess some of the earthworks, which appear to have once been channels, were constructed to divert water from the river to where it was required by the millers? (See also Crane River Park pages.)
I do love Donkey Wood it holds a lot of memories mostly good and I always enjoy a return visit.