I walked in Delhi’s oldest fort:
Delhi’s Red Fort, or Lal Qila, is not only an iconic structure but is synonymous with the city. However, much before the Lal Qila, there was another Red Fort in Delhi called Lal Kot, which was built by Anangpal Tomar .
It is the original Red Fort of Delhi. Let me clarify that Lal Qila was not built on the ruins of Lal Kot; they are quite far away from each other.

One is in Mehrauli while the other is in Shahjahanabad known as Puranik Dilli or Old Delhi.
We have documented proof for the foundation of Delhi from the first Tomar king onwards.

Raja Anangpal Tomar founded it in AD 736. He probably chose the rocky Aravalli hills in Mehrauli as his headquarters for the strategic and military advantages it offered. It was one of the reasons why Qutbuddin Aibak, the first Sultan of Delhi, and some of his successors continued to live in the Lal Kot/Qila Rai Pithaura area for some years till Kaikobad moved to Kilokhari.

Delhi was built over several years as successive rulers felt the need for fortification against enemies, or wanted to go closer to sources of water, or just wanted to create a magnificent city. The seven cities are still extant; the eighth city was designed and built by Edwin Lutyens.

The first city of Delhi — Lal Kot/Qila Rai Pithaura — is situated in Mehrauli. The seventh city of Shahjahanabad, where Lal Qila is situated, is 23 km away from Lal Kot. In between are the ruins of the five other extant cities: Siri, Jahanpanah, Tughlaqabad, Firozabad and Dinpanah/Sher Shah Garh.
The Tomars ruled over it from AD 736 but it was Anangpal II who re-peopled it and built Delhi’s first Red Fort in 1052-60CE. It seems probable that Anangpal II was forced to move to Delhi from Kanauj after the attack by Mahmud of Ghazni on Kanauj. Here, the Tomar kings, Anangpal and his successors, reigned undisturbed for a century, during which time they were able to build the city walls and construct masonry, dams, and tanks.
The excavations undertaken by the ASI in this area showed that the citadel was oblong in plan and “the high stonewalls to its west which enlarge the original enclosure and are usually regarded as its area are a later construction”. It also shows the presence of a palatial building.