The same Mughal emperor, Shah Jahan, who built Taj Mahal also built the Red Fort (which you will pass by but will need a whole half day to see inside; and also its 'sound and light show' is very impressive - giving the whole history).
Delhi is said to have been built and shifted seven times. The 8th time by Luyens who built New Delhi for the British rule.
In front of Red Fort (laal quila) is what we now call Old Delhi, surrounded by a fort like wall. Some parts of that wall still remain.
The wall has many gates, named after the trunk road to the city to which each one leads; for example, Ajmeri Gate road goes to Ajmer. So there is Lahori Gate for Lahore; Kashmiri Gate and so on.
Right in front of the Red Fort is the main road of Delhi called Chandni Chowk.
Chandni Chowk means Moonlight Square (Chowk is like circus as in Oxford Circus, Piccadilly Circus).
The street was built as main marketing centre. It can be called the oldest mall. It had trees on both sides and a canal in the centre. It ended at a pool of water in which moonlight would reflect. The British leveled the canal, cut down the trees and where the moonlight pool was they built a clock tower, ghantaa-ghar, to give the city a British character.
On both sides of the Chandni Chowk street are side streets and each side street has many sub-streets emanating from it. Each street and sub-street is specialized in a particular trade. For example, one street for jewelers and bankers. Shawl wali gali - specializing in shawls of all varieties. Parantha wali gali has now lost its real delicious variety of paranthas but it is still there.
The street foods even are specialized. There is a stall for extra large thick jalebis at an open air shop that has been there for several centuries.
If you started exploring the side streets, it will take days and weeks to explore everything. The large havelis (mansions) now dilapidated, unkempt, often with a water well in the courtyard, are now in dirty smelly streets, but still worth looking at.
I lived in Chandni Chowk area in an Arya-samaj building from 1949 to 1952 when I left India. I used to watch the daily hoisting and lowering of the national flag on the Red Fort from my upstairs room.
It is from the ramparts of the Red Fort that the prime minister of India gives his annual speech on Independence Day, 15th August. The Republic Day - 26th January - however is celebrated at India Gate, opposite the Parliament House.
I did a lot of naughty things in those teenage days: like letting Dhirendra (who shared the room with me) take me on his bike to Connaught Place for the first time taste of forbidden coffee. Did not like my first sip of coffee and could not then figure out why people drank such bitter brew!! - Until I tasted Cappuccino in Genoa a few years later.