Complex sentence: Alauddin Khilji started building the Alai Minar, who had doubled the size of Quwwat ul-Islam mosque, conceived this tower to be two times higher than Qutb Minar in proportioned with the enlarged mosque.
Split into simpler sentences: Alauddin Khilji started building the Alai Minar, after he had doubled the size of Quwwat ul-Islam mosque. He conceived this tower to be two times higher than Qutb Minar in proportion with the enlarged mosque.