Successive cities leave an unusually dense urban record
Delhi's archaeology ranges from prehistoric and late Harappan material to early historic inscriptions, medieval fortified cities and Mughal Shahjahanabad. The familiar “seven cities” is a useful convention, but it simplifies a longer and more continuous pattern of settlement.
Source: Department of Archaeology, Government of NCT Delhi — About Us
Delhi receives a National Capital Territory framework
The Constitution's sixty-ninth amendment and the 1991 government act created the National Capital Territory framework with an elected assembly and council of ministers. Delhi remains a union territory, with powers divided between its elected government and the Union under constitutional and statutory rules.
Source: India Code — Government of National Capital Territory of Delhi Act, 1991