[14][15] Simraongarh's relationship with the Simal forest has been also revealed by Gopal Raj Vamshavali, the oldest chronicles of Nepal.

The rule of the Karnat dynasty marks an important milestone and a golden age in the history of Tirhut.

[22] The rise of this empire saw the birth of efficient administration, social reforms, religious and the development of local folk music and literature.

[23][24] The Simraon, Karnat, or Dev Dynasty originated with the establishment of a kingdom in 1097 CE headquartered at present-day Simraungadh in Bara district.

According to French orientalist and indologist Sylvain Lévi, Nanyadeva established his supremacy over Simraungadh probably with the help of Chalukya king Vikramaditya VI.

[27][28][29] After the reign of Vikramaditya VI in 1076 CE, he led the successful military campaign over modern Bengal and Bihar.

[34][35] In 1877 AD, then Prime Minister Jung Bahadur Rana died at a place called Patharghat, Rautahat while returning from a hunting camp from Simraungadh.

It is also believed that the Maharaja of Oinwar dynasty, Siva Singh (r. 1412 - 1416),[38] built this palace for his daughter Isri Devi during his ruling time.

[40][41] The idol of Kankali Mai is placed in the temple, which is made of black stone and believed to be mangled by Tughlaq armies.

[43] The Jharokhar pond, which is connected to the Nepal-India border by Dasgaja, is believed to be about 800 years old and is located in Baswariya, Simraungadh.

[47] The Labyrinth and the powerful defense system of the city was well planned to protect from the river floods, enemies and regulate agriculture from controlled flow of water from the ditches shows the ability of the Kingdom.

During the reign of Rama Singh Deva (r. 1227 - 1285 CE) over Simraungadh, the Tibetan monk Dharmasvamin visited the fortified city in 1236 AD on his way back to Nepal and Tibet from Nalanda.