At least that seems to be the reason why the British decided to clear the area around the immediate vicinity of the Fort; it was now a major prize and had to be made unassailable. So, an esplanade was created, extending up to the point(s?) where de Lally's cannons were based (Sure, they did not account for technological advances...) and the new Blacks Town was created beyond those limits. A survey in 1772 fixed the boundaries of the esplanade by raising six obelisks, each rising about 20 feet high.
Only one remains; maintained by the Murugappa Group, it is painted in the same colour scheme as Dare House is; that is one reason why the passer-by will miss the inscription on the granite slab at its base, saying "Boundary of the Esplande, 1st January 1773"!
(click on picture to enlarge - the inscription can then be seen)