Question

Which weapon played a major role in the defeat of the Inca by the Spanish?

Answer 1

The horses and the superior weapons 

Answer 2

I believe the answer is: Artillery

Artillery could reach target in a much larger distance compared to normal fire arms. Since the long range weapons of the inca are limited to bows and arrows, the Spanish could easily destroy Inca's army from a safe distance, without fearing direct counter attacks.