Delhi, like India, is a fusion of disparate cultures over time. While several different settlements have been identified, most of the top surviving historical architecture are from the era of Delhi Sultanate, Mughal Empire and British Empire.
Delhi Sultanate, 320 years stretch of 5 unrelated dynasties, started in 1206, following series of events after defeat of the incumbent Prithviraj Chauhan (last ruler of the preceding empire) by Muhammad of Ghor. Qutb Minar, a minaret & victory tower, was built in this era and remains the tallest structure in India (73 meters). It is the earliest and most prominent example of Indo-Islamic architecture.
The Mughal Empire, founded by Babur, a descendant of Genghis Khan, ended the rule of Delhi Sultanate. Capital was shifted away from Delhi though reinstated by Shah Jahan, the 5th emperor. The emperor created Shahjahanabad, a walled city within Delhi. Two of the top monuments in India, the Red Fort and Jama Masjid, were built within this city. But, of course, the most impressive monument this emperor built was Taj Mahal (in Agra, about 200 kms from Shahjahanabad). Shahjahanabad is now called Old Delhi.
In the late 17th century, the Marathas rose in prominence as the Mughal Empire faded. British East India Co. defeated Marathas in 1803 & the capital of British India moved here from Calcutta in 1911. A new planned city was formed (New Delhi). Connaught Place, modelled after Royal Crescent of Bath (in England), is a major business and shopping hub. Some key attractions in New Delhi: Rashtrapati Bhawan, India Gate, Rajpath.
India’s independence (and concurrent partition) in 1947 resulted in significant changes in ethnic mix and subsequent influence on culture of Delhi. Prime examples of modern architecture in Delhi: IIT Delhi, Lotus Temple.
[I wrote this to serve as a guide for my friends visiting Delhi, one of my hometowns.]