Military strategy for the unmilitary

Someday I'll write a big, fat fantasy novel.  I say this, even though I don't particularly like reading big, fat fantasy novels.  Writers like to say that they write books they'd like to read.  I'm sure I could write a BFFN that I'd like to read--the question (and one that's come up a bit too frequently) is whether anyone else will want to read it.

But one thing I do know--BFFNs must have a lot of military activity, and at least one giant battle.  And it stands to reason that those battles will need to reflect the limitations and affordances of historical military combat.  Here is where I hope my deep reading in history will stand me in good stead.  I hope.

I'm not militarily minded and don't gravitate to explicitly military fiction.  But I do like to know how things got done.  Most history writers are either too fanatically detailed or too cursory for me.  What am I looking for?

In The Fall of the Roman Empire, Peter Heather gives an account of military operations after large Gothic forces crossed the Danube and moved into.  He describes how the Rhodope Mountains

...are extremely difficult to cross from north-east to south-west...and movement north and south through the Haemus Mountains is channeled through just five major passes....

And bravo for Heather, there is a nice clear map without a lot of extraneous details, showing the geography and the routes of the armies between 377 and 382.

He goes on, about the Romans.

Heavily outnumbered as they were, the available forces had no prospect of defeating the Goths; so...they fortified the passes through the Haemus Mountains...Some of the passes...are quite broad, but they are all high.

Then he gives an account of how 4,400 Russians held one of these same passes, Shipka Pass, against 40,000 Turks during the Russo-Turkish War of 1877.  For two months, the Romans were similarly successful, but then a force of Alans and Huns joined the Goths.  If one pass was forced, the soldiers at the others would be cut off.

Once the Goths and their allies were south of the Haemus, they could rampage at will.  The only geographic barrier beyond was the Hellespont itself.  This situation led to Valens's defeat at Adrianople.

It's this type of description of terrain and strategy that makes me feel that I understand something.  Using history as a crib, I could write some convincing strategic maneuvering, using what I know of geology, geography, and climate.  And I could make sure it was something I enjoy reading.  What about the rest of you?