What were the rules of engagement in military conflicts in the first centuries after Muhammad?

Did Islam have some rules of engagement in military conflicts, concerning women, children and old men? Were there any rules for not touching them after conquering a city or territory? Did Muhammad have such rules in his expeditions (military conflicts)? Rules change in time - for example in the Ottoman Empire they took non-Muslim children by force from their parents, periodically, to make them soldiers. So I am interested about the rules in the times of Muhammad and a few centuries after him.