This is the Alai Minar, which was started by Ala ud din Khilji and was intended to be double the height of the Qutub Minar and therefore has a base double the size of the Qutub Minar, but Al ud din died soon after the first storey was completed and thereafter construction wasn't pursued.
Just to give an idea of the scale, the Outer wall of this unfinished structure is around 15 feet thick. I climbed up the wall and went in to see what was inside. There was an inner structure half as thick as the Qutub Minar itself inside the 15 feet thick outer wall. I could just wonder what those guys were trying to build :-)
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