Nandi Hills or Nandidurg (Anglicised forms include Nandidrug and Nandydroog) is an ancient hill fortress of southern India, in the Chikkaballapur district of Karnataka state. It is located just 10 km from Chickballapur town and approximately 60 km from the city of Bangalore. The hills are nestled between the neighboring towns of Nandi, Muddenahalli, and Kanivenarayanapura. The hills are traditionally held as the origin of the Arkavathy river. There are many stories about the origin of the name Nandi Hills. During the Chola period, Nandi Hills was called Ananda Giri meaning The Hill of Happiness. Another story is that Yoga Nandeeshwara performed penance here, and so it was named after him. Nandi is also commonly called Nandidurga (Fort) because of the fort build here by the ruler Tippu Sultan. It is also perhaps called Nandi Hills because the hill resembles a sleeping bull (Nandi).