Naturally this camp hasn't really anything to do with the Danes, but is a hillfort from the Iron Age. When some of the site was excavated in the 1990s, various earlier artifacts were found too, so it's known the promontory was being used in the Bronze Age and Neolithic too. It's right on the cliff overlooking the Thames, so it only has earthworks on three sides. The SMR says "The site offers a clear vantage point onto the river, and wide views across the flood plain into Berkshire." This attractive spot wasn't overlooked by more modern settlers either, so that is why there is now a hotel there. Alas when the house was built c1900, they just flattened the western banks entirely and bunged the building on top. Apparently "A short section of the inner bank and ditch survives as earthworks to the south of the mansion, adapted in the early 20th century to serve as a rock garden with an ornamental walkway." So that's handy isn't it. You can even be taken on a tour of the gardens this summer, as part of the National Gardens Scheme.
My attention was drawn here by a totally unprehistoric but weird bit of folklore, about the Uncorrupt Hand of St James. Yep that's (allegedly) St James the apostle himself, Jesus's mate - here in Buckinghamshire. Who'd have thought it. His hand used to be kept in a chapel that was right here in the fort (the chapel, along with another house, got knocked down to be replaced by the present Danesfield House). Once upon a time it was kept in Reading Abbey and was a big draw for pilgrims. And today it resides at St Peter's church in Marlow, and you can see it there for yourself. There's a colour photo on Elizabeth Chadwick's blog, if you've got the stomach for it. I was reading about some of its adventures here in a 1901 book called 'Memorials of Old Buckinghamshire', by P H Ditchfield. So, not prehistoric. But says something about how we give meaning to and value the ancient past perhaps.
In the grounds of Robert Scott, esq. there is a strong and perfect Danish encampment in the form of a rude horse-shoe. In its circular part it is fortified by a double vallum; the front towards the Thames is defended by the high cliff.
A few years since some warlike instruments were found in making a walk round the rampart; but I have not had an opportunity of seeing them.
The place has been called by the country [people] the Danes Ditches, and has given the name of Danesfield to an elegant residence of Mr. Scott, who has improved the house and grounds with great taste.
From p835 of 'The History and Antiquities of the Hundred of Desborough', by Thomas Langley, 1797 (which can now be read on Google Books).