I was always a big fan of war stories and tactical RPGs, and one of these days I'd like to try writing a war story of my own. That said, I am currently working on smaller-scale character-driven stories, but who knows.... Maybe one of them will eventually evolve into something bigger...
EDIT: Also, to add to the previous topic... Just try to read as many historical accounts on war as you possibly can(if the said topic interests you). Study how war happens with and without guns, and how guns influence the tactics utilized in war. Also, MAKE A POINT TO STUDY BASIC MILITARY TACTICS. Youtube is chock-full of videos on the subject matter, so go there and study everything from Sun-Tzu to modern warfare.
That said, you're not going to complete your studies on war in a day or two. It'll take a while. So just start writing on the topic, and research whenever you're not sure about something. Make sure to study the subject matter on the side.
Once you're done studying the topic of war, I would recommend studying medieval trade, how cities were situated and built back at the time, as well as what common materials were used for crafting tools, clothes and buildings back in the day. The world view back then was wildly different, and even those world views varied wildly depending on the culture in question.
...Wow. I went full-on lecture mode on you. Sorry. This tends to happen when I get inspired... XD