What did west African kings become powerful by controlling

The west African kings:

The Kings in Western part of Africa by controlling the stops which were on one of the most important trade routes which was on trans Saharan trade routes.

The three African kings on the western sides which were Ghana, Mali and Songhai became very powerful even beyond their belief.

Because they had got over the control of one of the most important trade routes on the trans Saharan which led to the accumulation of their wealth. Gold and salt were the major commodities that were exchanged on this route.