Short answers, pulled from the story.
Archaeologists have observed that early Buddhist stupas were built near pre-historic megalithic burial sites associated with the Indus Valley Civilization. Broken Indus-era pottery was incorporated into later Buddhist burials at these locations, and scholars note structural features similar to pre-Mauryan-era cairn mounds.
Emperor Ashoka ruled from 273 BCE to 232 BCE according to Buddhist tradition and erected 84000 stupas to distribute the relics. The first known appearance of the word stupa is from an inscribed dedication by Ashoka on the Nigali Sagar pillar which spells the word thube in Pali using the Brahmi script.
The stylistic evolution of the Gandharan stupa became very influential in the later development of related forms in China Korea and Japan. Butkara Stupa received Hellenistic decorative elements from the 2nd century BCE while Loriyan Tangai stupas appeared in the 2nd century CE with elongated shapes and narrative reliefs.
The square base represents Earth, the hemispherical dome or vase represents water, and the conical spire represents fire. The upper lotus parasol and crescent moon represent air while the sun and dissolving point represent wisdom.
Phra Pathommachedi in Nakhon Pathom Thailand reaches 100 meters in height making it the second tallest extant stupa in the world. This type of tower-like stupa is thought to be the precursor of tower stupas in Turkestan and derives from the Gandharan tower-stupa model.