Although spoken Tibetan varies according to the region, the written language, based on Classical Tibetan, is consistent throughout. This is probably due to the long-standing influence of the Tibetan empire, whose rule embraced (and extended at times far beyond) the present Tibetan linguistic area, which runs from northern Pakistan in the west to Yunnan and Sichuan in the east, and from north of Qinghai Lake south as far as Bhutan. The Tibetan language has its own script which it shares with Ladakhi and Dzongkha, and which is derived from the ancient Indian Brāhmī script.
From what was the Tibetan language's script derived?
ancient Indian Brāhmī script