Originally built for the queen of Nepal in 1832, the lighthouse lookalike was rebuilt following a powerful 1934 earthquake that claimed more than 16,000 lives, according to the Wall Street Journal.
Open to visitors for the past decade, as many as 200 people were inside the nine-story tower when it toppled, a police officer told Reuters.
The Kathmandu Valley includes seven groups of monuments that showcase a range of religious and artistic traditions that have made the area world famous, according UNESCO. Among the most well-known are tiered temples made of fired brick and timber.
"The roofs are covered with small overlapping terracotta tiles, with gilded brass ornamentation," according to a UNESCO description. "The windows, doorways and roof struts have rich decorative carvings. The stupas have simple but powerful forms with massive, whitewashed hemispheres supporting gilded cubes with the all-seeing eternal Buddha eyes."
In the hours after the quake, rescuers picked through the rubble of historic structures around the region, including ancient wooden Hindu temples, according to Reuters.
"I can see three bodies of monks trapped in the debris of a collapsed building near a monastery," said Devyani Pant, an Indian tourist. "We are trying to pull the bodies out and look for anyone who is trapped."
The quake also destroyed buildings in Patan Durbar Square, a World Heritage Site founded in the third century that was known for it's exquisite Buddhist and Hindu architecture.
Pictures of damaged historic buildings circulated widely on social media. Eyewitnesses noted that many buildings have cracks and could still collapse in the coming hours as tremors continue to shake the region.