My first impression was that it was similar to 'Old Sarum' near Salisbury which was an iron age hill fort before various transformations by Roman, Saxons et all. However 'Old Sarum' has a distinctly flat top.
I'm guessing the Normans built on top of existing mounds to save time building mottes (me mum's idea not mine - but I agree)
I'm sure your mum's right. Some of the mottes I've seen here in Ireland, particularly one below the Slieve na Calliagh hills, have that air of the pre-historic about them.