This short film by UNESCO describes the historical significance of Lumbini as the birthplace of Gotama Buddha and its place as one of the Four Great Pilgrimage centres for Buddhists from all over the world.
There is an Asokan Pillar marking the site, which was set up in 249 BCE, which reads in part (mistakenly said to be in Pāli in the film):
King Piyadasi (Asoka), beloved of devas, in the 20th year of the coronation, himself made a royal visit, Buddha Sakyamuni having been born here.
Besides having an Asokan Pillar marking the place, recent archaeology suggests that the site goes back to the founding of the religion in the 5th c. BCE.
The second part of the film is about how to provide the present inhabitants near the site with a sustainable future, which includes both conservation and development.
The UNESCO listing of the site was made in 1967, and their website provides further information on the site and its significance.