Then put the TL on hiatus until you are. Weapons Fighting Techniques of the Medieval Warrior is a handy intro volume. In fact, it's one of the few books that sits on the "quick reference" shelf when I'm writing RoS updates. If you've got a local B&N that has, you can get it for $14. Another one is Battles of the Medieval World 1000-1500. Since it's an intro book you can essentially concentrate on why the strategy for the particular battle was chosen and see how the various pieces of medieval warfare work together.
Now do you need to write this out exhaustively? No, lots of people don't go into in-depth battles in medieval TLs like I do, but if you get an idea of what's actually possible you know how the story can go. For instance, Alfonso's campaign to capture Toledo and what actually happened in the Palestine Wa
r between Richard and Saladin after Arsuf, the second Siege of Acre, Phillip's siege of Chateau Gaillard (Cost to build the castle? Insane.) All these things will give you an idea of what is possible with appropriate preparation. As it happens I disagree in the particulars with Gustavus, Alexius for example, was actually fairly poor all things considered--he fixed the economy but they didn't have much money to spare until John. But yeah, not going to fall like that.
Oh cmon MNP why, I agree Alexius was not that great but his reforms were quiet useful, I agree he was not a good military leader cause he lost much of Anatolia but in the first crusade you can see the genius behind his plan making crusaders swear an oath, or how he succesfully reppelled the Norman invasion reformed gov and army, maybe he wasn't super great but his accomplishments are quiete incredible given the state of the empire at the time don't you agree, Byzantium was in a decline but he started a proccesa that temporarily brought it back.... So give him some credit.