The three hills of the place rise in the form of a triangle, all being steep, strewn with huge boulders which are largely unclimbable, and well fortified on every side by battlemented stone walls equipped with loop-holes for the use of guns and musketry. They are connected with one another by a stone rampart sixty feet thick and an external ditch eighty feet in width.
1. Rajagiri – The highest of the three hills, called Rajagiri, forms the principal fortification.
2. Krishnagiri – The northernmost of them is called Krishnagiri or the English Mountain so frequently referred to by the historian, Robert Orme.
3. Chandrayan Drug – The southern one is Chandrayan Drug or the St. George’s Mountain. The Chandrayan Drug is connected with Rajagiri by a low rocky ridge.
4. Chakkili Drug – A smaller and less important fourth hill is the Chakkili Drug (hence called Chamar Tikri by the 18th century annalist, Bhimsen), the summit of which was well fortified.
A steep flight of steps of hewn granite leads to the top of the first hill. The triangular space enclosed by these three hills forms the lower fort and the hills served as the citadels of the entire fort area. The lower fort between the hills was pierced by two entrances, one on the north by the Arcot or Vellore Gate and another on the east known as the Pondicherry Gate.