building. However, we do not find any batter in the buildings of Firoz:
Secondly, they attempted to combine the principles of the arch and the dome
with the slab and beam in their buildings. This is found in a marked manner
in the buildings of Firoz. In the Hauz Khas alternate stories have arches and
the lintel and beam. The same is to be found in some buildings of Firoz’s
Kotla fort. Finally, the Tughluqs generally used the cheaper and more easily
available greystone. Since it was not easy to carve this type of stone, their
buildings have minimum decoration.
                                 Hauz Khas fort
  Qutub Minar in red and buff standstone is the highest tower in India. It has
  a diameter of 14.32 m at the base and about 2.75 m on the top with a
  height of 72.5 m. Qutubuddin Aibak laid the foundation of the Minar in AD
  1199 for the use of the mu"azzin (crier) to give calls for prayer and raised
  the first storey, to which were added three more storeys by his successor