Number of armies and Citadels

[Fabiano]

The crusader part of the game is very confusing to me. Im not sure how many armies should I have or if i need to build citadels everywhere. I watched some guides, but no one explain these simple things, they tend to go on about units, resources, combat...

[Thariorn]

Citadels allow your captured Forts/Villages to defend themself longer against attacking demon armies.
If a fort falls, you have to recapture it and have to rebuild its buildings.

Amount of Armies is really dependent on your progress when it comes to demon armies and forts.

E.g., in Act III you can only explore so much of the worldwound and after clearing all static demon armies you only have to contend with the occasionally spawning attacking army
(Until your advance to the act III finale and ....stufff. happens)

I'd say have 2-3 advancing armies of roughly equal to strength to be able to clear the world map at a fast pace and one defending army staying close to Drezen in case of surprise attacks.

[Winger]

You don't need to build citadels everywhere. Building them in fringe zones are enough imo.

How many armies? Just one, led by a caster general. Damage spells trivialize army combat and are far more effective than any of your units. You can wipe out entire armies with a single cast of firestorm. I suggest the one named chief something, who starts with bonus to power stat and scorching ray spell.

What units? Doesn't matter but here's my own preference:
Ranged: Not marksmen, they get deleted by enemy general spells in one cast. Not rangers, "too tanky" for their role, trading too much damage. Rest are fine.
Caster/special: Assassins.
Cavalry: Hedge knights.
Infantry: Not shield bearers, rest are fine.

[The Cystocot]

For armies, i find 2 to be enough, both led by caster generals, Chief Abegoya & the Aasimar Countess are excellent generals, both get scrorching ray & fireball fairly early, as well as the cure wounds spell, absolutely decimating enemy units while frustrating their efforts to wipe your's out.
I generally kept one up near Palura's Fall & another down around the Tower of Yath settlement(south of Areelu's Lab). When random armies spawn, you have a general nearby to wipe them out, and they make exploration easier in the late game by clearing out all the new roads that open while collecting goodies & income buffs from the bad guys.

Slight spoiler: after you return from the events of Act 4, any armies you had will be effectively wiped out(not sure if this applies on the lich path, but my angel's army was decimated, thanks Queeny), so don't stress too much building massive stacks once your MC is Mythic rank 4, just build up enough to get the job done.

Finally, you eventually get a mass teleport skill for the campaign map, use it to keep topping up your southern army, while reinforcements for the PF army in the north just go out on foot, it's much, much faster to get fresh troops to the north bit of the map than the south...some damn long roads down there.

Oh, and ALWAYS take the trait to increase your army unit size on level ups!!
Good luck!