Question

when did british conquer shajahnabad?

Answer 1

After the fall of the Mughal Empire post 1857 revolt, the British Empire shifted the capital of India, to a more (security-wise) stable Calcutta (Kolkata), where it remained till 1911 (when they came back to Delhi). After quelling the 1857 revolt the Britishers built a military garrison inside the Red Fort and evicted the 3000 people (approximately) who were living there at that time and destroyed many of the residential palaces.
                  To be frank the city of Shahjahanabad (now better known as Old Delhi) never actually declined. The focus simply shifted from being an administrative capital city to being an area for trade and commerce (which it remains till date), with many large wholesale markets coming up in the mid 19th century. These markets like Chawri Bazar (hardware market est 1840), Phool Mandi (Flower Market est. 1869) e.t.c. exist till date.