The capital of Nepal, your gateway to the top of the world. If you take a flight to get in to Nepal, Kathmandu will most likely be where you land and make the preparation before trekking any part of Himalaya.
Ever been to India? My first impression of Kathmandu is that it feels like a mid-size Indian city, with more outdoor/hiking shops in the touristy areas.
There are plenty of religious and historical monuments to see in Kathmandu Valley area, seven of them are UNESCO World Heritage Sites. Many were damaged by the 2015 7.8 magnitude earthquake.