Assuming you have presence, can you play The Fires of War in battle? Does it depend on the stronghold? I ask because some strongholds don't affect the battlefield and from what I understand, you have to play cards that affect the battlefield or move a unit in.
The Fires of War Battle Action
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Assuming you have presence, can you play The Fires of War in battle?
Yes. Its Battle ability is not performed, it targets nothing and it does not come from a card in a unit, so the Rule of Location does not prevent you from playing it, and you say that you meet the Rule of Presence, so this is legal.
Does it depend on the stronghold?
No. It is legal even if your Stronghold has no Battle ability. It is legal even if your Stronghold has no ability at all.
from what I understand, you have to play cards that affect the battlefield or move a unit in.
No, the Rule of Location applies only to cards in a unit, and to targeting or performing. If your Battle action targets a card that is not in a unit, or affects a card in a unit without targeting it or being performed by this card, it is legal.