¤ An Observatory
The Jantar Mantar was built in 1710 by Raja Jai Singh II of Jaipur (1699-1743) in Delhi. This is an observatory consisting of mason-built astronomical instruments to chart the course of the heavens. Jai Singh, who was a very scholarly king with a very keen interest in astronomy and astrology, had other observatories built too – in Ujjain, Jaipur, Mathura (which no longer survives) and Varanasi.
The first among these was this one in Delhi. The yantras (instruments, which has been distorted to Jantar) are built of brick rubble and plastered with lime. The yantras have evocative names like, samrat yantra, jai prakash, ram yantra and niyati chakra; each of which are used to for various astronomical calculations.

Alai Darwaza
The southern gateway of the Quwwat-ul-Islam mosque was extended by Alauddin Khalji in 1311. The Alai Darwaza is the first through and through Islamic building in character, architecture and ornamentation. Its construction is painstakingly accurate, the decoration are the politically correct geometric designs and is a typical Khali building. It displays a certain Seljuqian influence, like in the huge dome with a central knob, pointed arches shaped like horseshoes, and the 'lotus-bud' fringes of the arches.

Alai Minar
The ambitious rubble Alai Minar started by Alauddin Khalji but the sultan lived to see it only the height of 24.5m. It was built to match the enlarged Quwwat-ul-Islam masjid (which was also Khalji's work). Today it is used more like an illustration, by parents, of what-happens-when-you-get-over-ambitious; viz the plans remain unfinished. Ambition has never really been encouraged as a virtue in India.

Qutub Minar in Delhi
¤ Constructed As A Holy Minar
Qutub MinarThe world famous towering Qutub Minar, started in 1192 by Qutub-ud-din Aibak (1192-98), breathes down the neck of the Quwwat-ul-Islam mosque. There is a slight difference of opinion as to its purpose: it probably was a tower of victory, but then again it could have been built to be a minar (tower), attached to the Quwwat-ul-Islam mosque, for the muezzin (priest) to climb up top for a prayer.