What structure on top of the stupa represented the abode of deities?

Harmika

The stupa, originating from the Sanskrit term meaning a "heap," began as a simple semi-circular earth mound known as an "anda." Over time, it underwent a transformation, evolving into a more intricate architectural form that skillfully combined circular and square elements. Situated atop the anda was the "harmika," resembling a balcony, symbolizing the dwelling place of deities. Extending from the harmika was a central mast called the "yashti," frequently crowned with a "chhatri" or umbrella. Encompassing the mound was a protective railing, demarcating the sacred realm from the secular world.

The early stupas, exemplified by those at Sanchi and Bharhut, exhibited a simplicity in their design. These structures featured stone railings resembling bamboo or wooden fences, as well as ornately carved gateways positioned at the cardinal points. Worshipers would traditionally enter through the eastern gateway, proceeding in a clockwise manner around the mound, keeping it to their right—a ritual mimicking the path of the sun across the sky. As time advanced, the stupa's mound underwent an intricate transformation, adorned with niches and sculptures as seen at sites like Amaravati and Shah-ji-ki-Dheri in Peshawar, Pakistan.