What role did the geography of the sahel play in the rise of the medieval african kingdoms?

it provided access to the large ports on the atlantic.

the lack of fresh water made rulers here more warlike.

it provided a place for trade between the north and the south.

the desert was difficult for outsiders to invade.

The right answer for the question that is being asked and shown above is that: "It provided a place for trade between the North and the South." The role that the geography of the Sahel play in the rise of the medieval African kingdoms is that it provided a place for trade between the North and the South. 

That is easy  the Twelve Tables (aka Law of the Twelve Tables) was a set of laws inscribed on 12 bronze tablets created in ancient Rome in 451 and 450 BCE. They were the beginning of a new approach to laws where they would be passed by government and written down so that all citizens might be treated equally before them.