Amar Singh Thapa takes Sirmaur

Amar Singh Thapa crossed the Yamuna and overran Sirmaur, driving out Karam Prakash II. Within two years the Gorkhas held the Shimla hills as far as the Sutlej — some thirty small states submitted.

Gorkha rule was remembered for its severity: heavy revenue demands, begar (forced labour), and the slave trade. This is the 'nature' of the invasion the syllabus asks about.