From Lonely Planet: "On the eastern side of Kathmandu...is the huge stupa of Boudhanath, the largest stupa in Nepal and one of the largest in the world. It is the religious centre for Nepal's considerable population of Tibetans and there are a number of thriving monasteries and many small shops selling Tibetan artefacts."
"Boudhanath has always been associated with Lhasa and Tibetan Buddhism. One of the major trade routes from Lhasa came through Sankhu, and Boudhanath therefore lies at the Tibetan traders' entry to Kathmandu. One can easily imagine the traders giving thanks for their successful journey across the Himalaya, or praying for a safe return. People still come here to pray before undertaking a journey in the Himalaya."
"There does not seem to be any agreement on how old the site is, but it is likely that the first stupa was build some time after 600 CE, after the Tibetan king, Songtsen Gampo, was converted to Buddhism by his two wives - the Nepali princess Bhrikuti (sometimes regarded as an incarnation of the Green Tara) and Wencheng Konjo from China (the White Tara). The current stupa was probably built after the depredation of the Mughal invaders in the 14th century.
Stupas were originally built to house holy relics, or to commemorate an event or place, with a structure that symbolizes Buddhist beliefs. They are never hollow. It is not certain if there is anything interred at Boudhanath, but some believe that there is a piece of bone that once belonged to Gautama Buddha.
The base of the stupa takes the shape of a mandala (symbolizing earth); on this four tiered base sits the dome (symbolizing water); then comes the spire (symbolizing fire); the umbrella (symbolizing air); and the pinnacle (symbolizing ether). The Buddha's watchful eyes gaze our in four directions from the square base of the spire. There is a third eye between and above the two normal eyes and the 'nose' is not a nose at all but the Nepali number one, signifying the unity of all life. The spire is made up of 13 steps, representing the 13 stages on the journey to nirvana.
Around the base of the stupa's circular mound are 108 small images of the Dhyani Buddha Amitabha. A brick wall around the stupa has 147 niches, each with four or five prayer wheels bearing that immortal mantra om mani padme hum. On the northern end of the stupa is a small shrine dedicated to Ajima, the goddess of smallpox."