As a young child, Amina was caught playing with a dagger, holding it like a soldier would, which should have told somebody something. Then, when she grew up, she trained with the military so that after she became Queen of Zazzua (now the province of Zaria in northern Nigeria), she led her own forces to spread her empire and influence throughout the region.
Since the Zazzuans were adept at tanning leather, weaving, and metalworking, it's likely that Amina's troops wore armor, including metal helmets. Additionally, she was responsible for having walls built around the cities and military camps in her kingdom -- still standing in some places and still called "Amina's walls."
Convinced that to take a husband would weaken her power, Amina chose, instead, to pick out a lover from among her vanquished foes every time her troops won a battle. Then, the following morning, she would have the lover killed. She died, not surprisingly, in battle, said to be "a woman as capable as a man." As if that's difficult.
