You see, India is a nation that is usually peaceful, and setting that aside, the country back then was quite powerful and rich so invading foriegn lands was not promising, It is also surrounded on three sides with large water bodies and at the north by the world's largest mountain ranges-The Himalayas and The Karakoram. As an addition, the Indian kings were too busy fighting amongst themselves to care about what went on in the world outside.
Gateway to the Great Stupa at Sanchi was originally commissioned by the emperor Ashoka-the-Great in the 3rd century BCE. Its nucleus was a simple hemispherical brick structure built over the relics of the Buddha. It was crowned by the chatra, a parasol-like structure symbolising high rank, which was intended to honour & shelter the relics