Tree Shaped Tombstones - Chain and Spikes

I'm getting ready for England, so no better time to publish a couple of  UK "Tree Shaped Tombstone" images sent to me by roving correspondent "Chris J."  Thanks, Chris!

I'm not sure exactly where these were taken but the style "Rustic Cross" is what you generally find in Old World monuments of this class.  The "Rock of Ages" base is also typical.

Naturally I can't look at any photo without wondering about things.  This next one for instance.  Having posts around the perimeter of a grave is common, although often they mark out a larger family plot with several graves within.  But what was the purpose of the chain and spikes?  

I suppose it is metaphorical, just a symbolic way to separate the lands of the quick and the dead.  It seems a bit excessive for instance if it was a practical measure to keep a Grieving Maiden from sobbing on the slab in an embarrassing fashion.  And if you wanted to keep your Disgruntled Business Partners from dancing on your grave well, they'd stand at the foot of it, ponder a moment or two and come up with an even worse option......

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Addendum: Highgate Cemetery, London.