I have always been fascinated by this piece of colonial architecture. In fact, its a building to marvel upon - no pillars to support the building with a spiral staircase all round it for people to climb up and down.
The building was built by the British in 1786 to store grains for the British Army to protect them from famines. In 1770, a huge famine had killed millions of people and hence the British decided to make a building where they could simply store grains. The idea was simple, porters will climb up the building and drop the grains from the top.
Today, it serves as a picnic spot. A must visit if you are in Patna. Make sure you climb on top (if you're not afraid of heights). You can read more about the Gol Ghar here.