Buddhist religious architecture developed in South Asia in the 3rd century BCE. Three types of structures are associated with the religious architecture of early Buddhism: monasteries (viharas), places to venerate relics (stupas), and shrines or prayer halls (chaityas also called chaitya grihas), which later came to be called temples in some cases. Viharas initially were only temporary shelters used by wandering monks during the rainy season, but later were developed to accommodate the growing and increasingly formalised Buddhist monasticism. An existing example is at Nalanda (Bihar). A distinctive type of fortress architecture found in the former and present Buddhist kingdoms of the Himalayas are dzongs. The initial function of a stupa was the veneration and safe-guarding of the relics of the Buddha. The earliest surviving example of a stupa is in Sanchi (Madhya Pradesh). In accordance with changes in religious practice, stupas were gradually incorporated into chaitya-grihas (prayer halls). These reached their high point in the 1st century BC, exemplified by the cave complexes of Ajanta and Ellora (Maharashtra). The Mahabodhi Temple at Bodh Gaya in Bihar is another well known example. The Pagoda is an evolution of the Indian stupa.