Tribhuvan International Airport (TIA) in Kathmandu is the only international airport in Nepal. TIA has direct air links with Osaka, Shanghai, London, Frankfurt, Hong Kong, Singapore, Bangkok, Delhi, Dubai, Bombay, and Calcutta.
If you are traveling from other cities, you can easily find a connecting flight to Kathmandu as many airlines such as Fly Dubai, Etihad, Qatar Airways, Royal Nepal Airlines (RNAC), Air India, Singapore Airlines, Thai Air, and Thai lion run daily flights to and from Nepal.
You cannot use water transport since the country is landlocked, but there are land routes from different parts of India, Tibet, and China. Therefore, you can enter Nepal through the checkpoint on these land borders.