Answer:
By Conquering other kingdoms.
Explanation:
After winning the war, the kingdoms will take ownerships over the land and resources that exist in the losing kingdom. (even enslaving the citizens of the losing kingdoms was a common practice in this time period) This will massively increase the wealth of the winners and make the rulers become more powerful.
This practice stopped in 1600s , after European empires developed navigation technology and realized that colonizing weaker countries on another continent was more beneficial for them.
