Ajmer
Hidden behind the Dargah, this extraordinary 12th-century mosque built in just two-and-a-half days (hence the name) is one of India's earliest examples of Indo-Islamic architecture — its ornate Arabic calligraphy-carved arches and delicate pillars looted from Hindu temples create a fascinating and complex historical site.
This site is greatly undervisited despite being architecturally extraordinary — go early to explore without crowds.