Taking over fortressses before weakening changes quests?

In Far Cry 4 I just took over De Pleur's fortress. It was still marked as hard (weirdly enough I found it way easier than the average 'easy' outpost). I have heard I'm supposed to do some quests to weaken the defences of the fortress before attempting to take it over (unless you're feeling courageous).

Does taking over the fortress before weakening it's defences unable me to do those quests? How do I recognize quests that weaken the fortresses defences? Are there any benefits to taking over the fortress early?

I see a Longinus quest on the map nearby the fortress. Not sure if it has anything to do with it since I haven't done it yet. I assume quests to weaken the fortress are either unskippable story missions or unnecesary side quests.

Solution 1:

Without giving away too many spoilers, there shouldn't be any restrictions or punishments for taking down a fortress while the difficulty is "hard". Longinus quests have nothing to do with fortresses. The story missions would eventually weaken the fortress but there's nothing stopping you from doing them early.

The main benefit to taking over the fortresses is that outposts in the area won't be subjected to random take-over attempts.