his = Bhagwant Singh, am I right?
If I am right, why would he (Bhagwant Singh) dismantle his own fort?
Upsurge in Hathras (1817)
Dayaram, a talukdar of several villages in the district of Aligarh,
had a strong base in the fort of Hathras. The fort, considered to be
among the strongest in India—a ‘second Bharatpur’—had walls of
great height, and thickness, defended by a deep ditch and artillery
mounted at the top. The English had concluded the settlement of
Hathras estate with Dayaram as a farmer. But due to progressively
increasing high revenues, Dayaram constantly failed to pay arrears and
even committed many acts of hostility by giving harbour to government
fugitives. So, the Company with a large army attacked Hathras in
February 1817. Dayaram fought bravely for more than 15 days and
escaped unharmed. But, ultimately, he was obliged to come back on
condition of submission and settled down with a pension. Another noted
rebel Bhagwant Singh, Raja of Mursan, frightened to dismantle his
fort, submitted to the government. (source)
It is not very idiomatic. I’d read it as saying that Bhagwant Singh was afraid that he would have to dismantle his fort if he resisted the Company forces. So he surrendered without a fight. This assumes the author has used some non-idomatic phrasing.
It is possible that the author intended to say that Bhagwant Singh was frightened into dismantling his fort (by seeing the harsh treatment of his neighbour) and (having dismantled it) submitted. Again this reading assumes that there are some errors in the phrasing.