This raised dais used to have a relatively modern circular open temple on it, however it was deemed unsound and demolished in 2011.
The whole of the Belvedere (meaning beautiful view) hill area dates from the park's restoration. This style of woodlands and vistas or rides is a restoration of the layout of the entire area surrounding the castle in the 1750s prior to the remodeling which was completed by 1802. General Roy's map of 1747 - 52 shows that the ornamental woodlands were a series of these radiating rond-points of different sizes, sometimes overlapping each other.
There reputedly used to be a secret army bunker under the mound, however since the recent landscaping there is no sign of it. Maybe you will get lucky and find it!