To reach it these days you need to take a right hand fork in Mt Austin Road just before you get to Mountain Lodge gate House but Streetview tells me there is a small sign saying "No Admission, Private Area", so I can only assume it is related to the nearby reservoir or radio station on the peak.
Carter Wong with the radio station behind him
The 1970's Wanchai reclamation in the background
But why the circle at the top? I can only assume that at some point in the past before it was made off limits it perhaps housed a small pavilion. This next picture shows the view to the south with the unmistakable garden pavilion in the background (and the slope of the other aforementioned hill on the left). For those that didn't realise, the pavilion and raised terrace marks the position where the original Governor's Mansion used to stand.