What are some major characteristics of global cities?

I think that this can be answered in multiple ways because there is not a right answer to this specific question. But I think that some characteristics is that they have be industrial, because if a place can not produce then it can not stand, and I also think that it is defined by how much power the city has economically and physically producing.

Culture, government and economics. You can base your questions on these topics to show why it's a major characteristic of the city.