Why it's the capital
In 1768 Prithvi Narayan Shah, king of Gorkha, entered Kathmandu unopposed while the city was celebrating the Indra Jatra festival. He made it the royal capital of the unified Kingdom of Nepal, which he founded.
Capital of NepalKathmandu sits in central Nepal, in the northwest part of the Kathmandu Valley and north of the Bagmati River, about 1,320 m above sea level.
In 1768 Prithvi Narayan Shah, king of Gorkha, entered Kathmandu unopposed while the city was celebrating the Indra Jatra festival. He made it the royal capital of the unified Kingdom of Nepal, which he founded.
Nepal is the long, narrow country between India and China. Kathmandu is a little east of its middle, about halfway between the Indian and Chinese borders.
Sources: Wikipedia: Kathmandu, Wikipedia: Prithvi Narayan Shah, Wikipedia: Kasthamandap, Wikipedia: Swayambhunath, Wikipedia: Boudhanath