Udayagiri Fort is located about 100 kms from Nellore atop a hill at a height of 3079 feet. It boasts of great natural beauty with lush vegetation and beautiful waterfalls. Access to fort is only by steps and there is no road to the fort.

During the rule of the Gajapathis and the Vijayanagar Rulers, this town grew to great splendor. The ruins of 365 Temples found in the town reflected the sculpture and architecture of various periods. The structures on the hill such as the Ranganatha Temple reflects Chola culture, the Balakrishna Mandiram reflects Pallava culture and the Paruveta Mandapam reflects Vijayanagara culture. Other structures include Chinna Masjid and Pedda Masjid.

The hill of Udayagiri is also popularly known a Sanjeevani Hill as there are many medicinal plants on the hill.

Here is a good article on a trek to Udayagiri Fort by Chennai Trekkers