While expose the Qutub Minar Complex, I have noticed that the worker and architect of that time , waste rock or Stone (which are the unused pieces or broken at the time of engraving or cutting) mixed with the solution (cement at those days) and form the interior structure and the beautiful engraving , design and Quran Aayat were stick from the outer .
This would provide the strongest structure at that time which protect the whole infrastructure and also a strong base to support the engraving and design .
According to me, all these historical structure stay long undistributed building and also bare the natural disasters and whether variation so long .
Incredible India
All the aforesaid matter are only my expectations but may not be true.
I have stroke this idea when I have seen the picture below
