Any simple tips on retinue composition?
I read in the past some things about it which appeared too complex for my mind.
I'm playing as Lombards turned Italian c.860AD. I have a retinue of ~2,000 units in one army.
Should I keep each unit type in a separate flank (i.e. cavalry left flank, light infantry center, HI right) or mix them together?
Should i keep a separate army for each unit type?
One commander per flank or one for all 3 flanks?
Any reasoning for above?
It's some reading work, but you are best off building your retinue so that you can only get a limited amount of tactics, and can do a lot of damage with those tactics.
https://ck2.paradoxwikis.com/Combat_tactics
For example, you can build a strong retinue by combining one Shock, one Defense and one Skirmish retinue, which are all three retinues available to everyone. This gives an army composition of 250 Pikemen, 250 Heavy Infanty and 300 Archers, or a multiple of that, which is about 31%, 31%, 38%.
On the combat tactics page you can see that this combination does not allow any harrass tactics, which you don't want if you don't have a lot of cavalry, but which can be picked even if you have only a little bit of cavalry (this is why you do not mix retinues with levies if you have any significant number of retinues). It
does allow Volley Tactic, which makes archers (who are strong in skirmish phase) stronger. Swarm tactics are unavailable because of no horse archers, and of the Defense tactics, it allows the Shieldwall tactic. And
that is the one you want, as it significantly boosts both Heavy Infantry and Pikemen, and also boosts Archers to a degree. If you check the multipliers of Shieldwall and Volley, you see that, with the army composition you have, Shieldwall becomes
much more likely - the total probability, assuming a 16+ martial commander, is 243, compared to about 10 for Volley, and insignificant numbers for a very limited few other tactics.
Then, in the melee phase, the possible tactics are Advance Tactic (great for Infantry, bad for Pikemen, likelihood 4.5), Stand Fast Tactic (opposite of Advance Tactic, great for Pikemen, bad for Infantry, likelihood 4.5), Slow Advance (great for Pikemen, good for Infantry, likelihood 4.5), Barrage Tactic (great for Archers, likelihood 9) and Force Back (great for Pikemen and Infantry, bad for Archers, likelihood about 10). This is more of a mixed bag, but
all tactics are positive on average due to lack of clutter that enables tactics you don't want (e.g. if you have 10 Heavy Cavalry clutter in this composition, you have a 3 likelihood to roll Powerful Charge, which makes all your Pikemen and Heavy Infantry useless), and the two best tactics (Barrage Tactic and Force Back) are the two that are most likely. On top of that, the balanced composition means that the difference in value between Advance Tactic, Stand Fast Tactic and Slow Advance is small, so you have a reliable strength. It won't vary wildly based on dice rolls. Note that all likelihoods assume a 16+ martial commander.
This may very well not be the best retinue composition (I'm not that experienced myself), but it's a relatively simple to remember retinue that reliably does it's job. Simply get one each of the retinues that include either pikemen or heavy infantry. 800 units per composition, total retinue cap cost of 1975, so it's even easy to calculate how many full sets you can train. Note that you typically don't want to mess up the composition, as the pikemen or heavy infantry numbers dropping under 30%, or even just becoming uneven, may make certain tactics (significantly) less likely. What
may be an option is to increase the amount of archers slightly, as that disables Volley Tactic, meaning Shieldwall Tactic is pretty much the only option in the skirmish phase, but this will likely cause an imbalance in pikemen and heavy infantry numbers.
A more general advice on army composition: Less different types of units in a flank tends to be better, and even 1% of a unit type can mess up your tactics.