I clicked this photo from one of the many viewing points in Amer Fort. You can see the Aravalli Hills in this photo. Beginning in Gujarat, near Ahmedabad, the Aravallis stretch across Rajasthan in a north-easterly direction, ending in Delhi.
The Aravallis are India's oldest mountain range. They define the ecology of Rajasthan, by providing a barrier to the Thar desert; they prevent the desert from growing. Several rivers originate in the Aravallis.
Many of Rajasthan's superb forts are located on hilltops in the Aravallis. Last year six of them were declared UNESCO World Heritage sites: Chittorgarh, Kumbhalgarh, Ranthambhore (Sawai Madhopur), Jhalawar, Jaipur, and Jaisalmer. I've marked them out on a map below.