What is stupas?

The stupa (a Sanskrit word meaning a heap) originated as a simple semi-circular mound of earth, later called the anda. Gradually it evolved into a more complex structure, balancing round and square shapes. Above the anda was the harmika, a balcony like structure that represented the abode of the gods. Arising from the harmika was a mast called the yastii, often surmounted by a chhatri or umbrella. Around the mound was a railing, separating the sacred space from the secular world outside.
The early stupas at Sanchi and Bharhut were plain except for the stone railings, which resembled a bamboo or wooden fence, and the gateways which were richly carved and installed at the four cardinal points. Worshippers entered through the eastern gateway and walked around the mound in a clockwise direction keeping the mound on the right, imitating the sun’s course through the sky. Later, the mound of the stupas came to be elaborately carved with niches and sculptures as at Amaravati, and Shah-ji-ki-Dheri in Peshawar (Pakistan).