Prithviraj Chauhan built Qila Rai Pithora in the 12th century as the capital, extending the original fort city of Lal Kot built by the Tomar Rajputs. The ruins of Qila Rai Pithora's ramparts can still be seen near Qutub Minar and Mehrauli. Alauddin Khilji of the Khilji dynasty established the fort city of Siri and had the Hauz Khas madrasa constructed, showing West Asian influences. In 1330, Ghias-ud-din Tughlaq founded the Tughlaq dynasty and built the fortified city of Tughlakabad to defend against Mongol invaders

1. http://www.india-delhihotel.com

2. QILA RAI PITHORA
The capital of Prithviraj Chauhan, Qila Rai Pithora, was the Delhi built
in the 12th century as per available historical records. It is said that his
ancestors had won the city from the Tomar Rajputs, who are credited
with forming Delhi. Anangpal, a Tomar ruler, created the first known
regular fort here and called it Lal Kot. Prithviraj Chauhan extended the
city beyond the fort. The ruins of its ramparts are still visible around
Qutub Minar and Mehrauli.

3. The fort city of Siri
The Khilji (or Khalji) rulers followed the Slave Dynasty. The most
prominent among the six rulers was Alauddin Khilji, who extended his
kingdom to the south of Narmada and also established the city of Siri.
Among some of the remaining ruins is part of the Siri Fort in the greater
Hauz Khas area. The madrasa at Hauz Khas was constructed during
Alauddin's reign and bears the stamp of West Asian architecture.

4. TUGHLAKABAD
In 1330, Ghias-ud-din Tughlaq established the Tughlaq dynasty and set
out to rebuild a Delhi of his own. His dream was to build an impregnable
fort to keep away the Mongols. Thus he founded the city of Tughlakabad,
the ruins of which still remain. His descendant Muhammad bin Tughlaq
later formed Jahanpanah. Lying in the area between Siri and Qila Rai
Pithora, this was the fourth medieval city of Delhi built in AD 1326–27.