Around 250 B. C., thousands of stupas were built in the Gupta Empire. What is a stupa?

A Stupa is actually a dome shaped Buddhist monument or we can say a hemispherical Buddhist structure which houses the relics of Buddha. These relics were worshiped by the followers of Buddhism. Another Buddhist monument called Chaitya or prayer hall usually contains a stupa. Thousands of stupas were built during the ancient India.

A dome-shaped structure erected as a Buddhist shrine.

That is what I got