Garrisons are Mandatory

[Arima]

In Shogun 2, Rise of the Samurai, and Fall of the Samurai, any and every Computer-Controlled Faction that has a Navy will threaten you with the prospect of an Amphibious Landing. Therefore, if your Castles are not properly developed and sufficiently manned, you're effectively asking for an enemy castle assault that you have very little possibility of winning against.

There is also the danger of a provincial revolt if you don't have the proper infrastructure and a sufficient amount of troops to prevent Public Order from going into the negative.
This problem is very true in both Rise of the Samurai and Fall of the Samurai.

You're wondering "If I really need to leave some units behind to defend my provinces, what units should I use then?"
Here are my best recommendations:

Shogun 2
You will need at least 1-3 Katana Samurai with Exceptional Armour to prevent an Enemy Siege Assault from being successful.
Also, 6-8 Bow Ashigaru with Exceptional Accuracy are sufficient enough to slow down and weaken the attackers.

As the Ikko-Ikki, you can substitute Katana Samurai and Yari Ashigaru with Loan Sword Ashigaru instead.
At least 6 of them with Exceptional Armour is sufficient enough to beat a Full-Stack Army.

Rise of the Samurai
"Resistance to Invaders" becomes a Very Serious Problem if you acquire Provinces through Military Conquest instead of Requesting their Allegiance with Junsatsushi. Eventually, you will reach a point in your Campaign where every Province captured will have a Permanent -4 "Resistance to Invaders" Penalty, making Garrisons mandatory.
Minimum Garrison: 4-6 Bow Levy with Exceptional Accuracy and 2 Naginata Levy with Exceptional Armour
Optimal Garrison: 4-6 Bow Attendants with Exceptional Accuracy and 1-3 Sword Attendants with Exceptional Armour

Fall of the Samurai
Minimum Garrison: 4 Line Infantry with Exceptional Accuracy
Optimal Garrison: 6-8 Line Infantry with Exceptional Accuracy and 1-2 Katana Kachi

[6Bagheera9]

The AI clans are pretty predictable in term of what provinces they are going to go after. Some are obvious because of the geography, but you can always figure out which need to be defended because the AI will repeatedly attack the same provinces. Thus if one provinces has been attacked once or twice, you can be fairly certain that some of the other ones are fairly safe and probably don't need many extra garrison troops.

I feel that 6-8 bow ashigaru are a bit excessive as it will be difficult to find space for all of them on the walls facing the enemy. Yari ashigaru are also cheaper alternative to Katana samurai and in spear wall are quite effective at containing attackers if positioned just behind the ramparts or at gates. I also think that if you really feel that a samurai garrison is necessary, and sometimes it is, Naginatas do the job better. The defender usually wins by exhausting the attacker rather than hacking through all of them and Naginatas excel at jamming up choke points to this effect. They are also particularly resistant to arrow fire which can be a huge problem if the AI bring many archers (very often.)

Lastly, matchlocks of any kind are brutal when firing from the upper level of a castle onto the lower patios. 2 matchlock ashigaru will make a castle much harder to assault and cost relatively little.

[O'Hea]

I'm with Bagheera, once you get the tech for it teppo ashigaru are easily superior to archers and in many situations naginata sams are preferable to katanas.

[RAFAEL]

I totally agree with Bagheera and O'Hea but I can also add that keeping a good fleet near your shores can save you a lot of trouble and prevent amphibious landing behind your lines. I can't count how many times I sunk full stack invasions with my stack of fleet, it also allows me to focus my units on the frontline.In FOTS public order is of an issue due to the modernization however a nice building combo for cash-provinces would be the inn line along with cottage industry line as for the military provinces you can go 2 ways, either go with castle upgrades to increase public order (with the benefit of better garrison) and military buildings OR go with the inn line along with whatever military building you want. I usually go with the castle upgrade when I find a worthy province (with blacksmith) or when I need a military province on the frontline or good geographical position (N.Shinano province for ex). Most of the times I build an inn just to maintain public order with less garisons..

[daniu]

I find it more cost-efficient to have minimal garrison in my cities and retake them if they get conquered.
Shogun 2 I have like 1 Yari, 1 Bow Ashigaru and in FotS I use 1 Levy Inf 1 Levy Spear. Military production cities get a bit more (2/2) so they don't get overrun.
Upon a naval invasion I draw all troops together and build some Samurai for support, and usually retake within a few turns.

Then again, like RAFAEL says, I do maintain a fleet and most of the time manage to intercept invasion forces.

[Arima]

Shogun 2 I have like 1 Yari, 1 Bow Ashigaru and in FotS I use 1 Levy Inf 1 Levy Spear. Military production cities get a bit more (2/2) so they don't get overrun.

I know from experience that Militia Units are absolute rubbish at both attacking and defending, even when you have a lot of them.

The only thing that they're actually good for is being the sacrificial lamb: I leave them at the first level of a Castle to hold up the enemy while my ranged units shoot away at them. It can only work as long as you're not defending against a full-stack army that has sword units.

[daniu]

I know from experience that Militia Units are absolute rubbish at both attacking and defending, even when you have a lot of them.

Well firstly no, I have managed to defeat two full stacks (of about half Samurai, half Ashigaru) with a half stack of Ashigaru in castle defenses.

Secondly, I don't seem to have made my point clear enough. The higher tier units cost about 150 upkeep per turn, low-tier ones 80, so about half. That means that keeping a garrison of 2 Ashigaru instead of 4 Samurai is a quarter the price (160 to 600). That's a difference of 440 per province (and with three provinces an additional low-tier building, bringing in additional funds) per turn in which no naval invasion strikes.
Now in case of an invasion, let's say the reduced garrison means I lose a province or two more. For high-income building with a high tax rate, let's say 4000 wealth and 30% tax, that's 1300 per province per turn; so for every province I lose I just need to not lose three others just to break even - and that's pretty much worst case scenario (because my 4000 wealth cities do usually have higher tier castles and more garrison so I'm not going to lose them in the first place). Plus like I said, reconquering is easy so everything's back to normal after two or three turns.

[Cruzz]

Well firstly no, I have managed to defeat two full stacks (of about half Samurai, half Ashigaru) with a half stack of Ashigaru in castle defenses.

Ashigaru are better than levies, when you compare how well they fight against samurai versus how well levies fight versus the "proper" fots units. Plus levies in FOTS are pretty high cost in upkeep (or rather proper unit upkeep is much lower than in sengoku even though the levies are the same ~80 per turn on the higher difficulties)

That said, I wouldn't keep garrisons in every town (especially not in fots where you control where the AI lands with port upgrades anyway), I'd only keep a slightly larger one in some province I don't want to lose even momentarily from which it is fairly fast to get to the other ones should they get invaded. It really is a waste of upkeep money unless you can run higher taxes constantly thanks to those garrisons.

[Arima]

That said, I wouldn't keep garrisons in every town (especially not in fots where you control where the AI lands with port upgrades anyway), I'd only keep a slightly larger one in some province I don't want to lose even momentarily from which it is fairly fast to get to the other ones should they get invaded. It really is a waste of upkeep money unless you can run higher taxes constantly thanks to those garrisons.

In FOTS, if you have a lot of Provinces, get them well developed, and research the right technologies, your Clan's economy can jump upwards to 10,000+ Koku per turn, making upkeep a completely irrelevant problem for maintaining garrisons.

[6Bagheera9]

A small defense fleet is another very good idea that I sometimes use. The trick is spotting the invasion fleet early enough. The AI usually transports armies with one or two ship fleets(coding flaw imho) and its very easy to sink those with a couple of medium bunes.

[weirdoascensor]

Not to say that Castles provides morale boost and a possibility when cornered, Fight to The Death.

[Gnufan]

In a castle battle a defending yari ashigaru unit can even defeat katana samurai by its own. The samurai retainer garrison can defeat multiple ashigaru units on its own. If you put 2 or 3 units of yari ashigaru in a castle you will be able to defend against a much larger force as long as you can lure their archers up the walls by staying outside their range. Even if the province is lost you can do some serious damage to the enemy army with just a handful of men. Spearwall and charges are key.

In ROTS I recommend foot samurai for defending provinces since they are the best melee unit as well as the best archers. In FOTS it's easy to defend with a small force as long as you have a castle with multiple levels. If you don't you want at least one unit of elite melee infantry such as the katana kachi or shogitai to support your gunners.

[6Bagheera9]

In a castle battle a defending yari ashigaru unit can even defeat katana samurai by its own. The samurai retainer garrison can defeat multiple ashigaru units on its own. If you put 2 or 3 units of yari ashigaru in a castle you will be able to defend against a much larger force as long as you can lure their archers up the walls by staying outside their range. Even if the province is lost you can do some serious damage to the enemy army with just a handful of men. Spearwall and charges are key.

In ROTS I recommend foot samurai for defending provinces since they are the best melee unit as well as the best archers. In FOTS it's easy to defend with a small force as long as you have a castle with multiple levels. If you don't you want at least one unit of elite melee infantry such as the katana kachi or shogitai to support your gunners.

I generally agree with everything you have to say, but I would not feel too confident in trying to hold off Katanas with Yari Ashigaru alone. If the katanas have comparable upgrades and xp compare to the ashigaru, they will probably beat them. I might see it happening if they katanas took many loses from missile fire beforehand. I would feel much better if I had two ashigaru per katana or retainers that charged the katanas from the flank while they were trying to hack through a spear wall.

[O'Hea]

A bit of a tangent, but if you're Ikko you can usually get by with nothing but your Temple-spawned monk garrisons and some loanswords. They have it easy garrison-wise.

[Arima]

A bit of a tangent, but if you're Ikko you can usually get by with nothing but your Temple-spawned monk garrisons and some loanswords. They have it easy garrison-wise.

Speaking of the Ikko-Ikki, they have the exclusive benefit of recruiting Loan Sword Ashigaru which are cheaper than Katana Samurai and more reliable in melee than Yari Ashigaru.
If you use Loan Sword Ashigaru for anything else other than Defending Castles and fighting Yari Ashigaru, their poor morale will do them in.

[6Bagheera9]

Speaking of the Ikko-Ikki, they have the exclusive benefit of recruiting Loan Sword Ashigaru which are cheaper than Katana Samurai and more reliable in melee than Yari Ashigaru.
If you use Loan Sword Ashigaru for anything else other than Defending Castles and fighting Yari Ashigaru, their poor morale will do them in.

They work fine as a general purpose flanker if they have a couple of levels of xp and a general with minimal bonuses to troop morale. Sure they'll run before Katana Samurai will, but if you put them in such a situation they would probably die even if they stood their ground.

[Gnufan]

Worst thing about loan sword ashigaru imo is that they are so stupidly strong in autobalance because of their bonuses against spear units. One of the reasons the Ikko Ikki AI often steamrolls nearby factions.

[Cruzz]

Worst thing about loan sword ashigaru imo is that they are so stupidly strong in autobalance because of their bonuses against spear units. One of the reasons the Ikko Ikki AI often steamrolls nearby factions.

???

They are always undervalued by the autocalc, as are all sword units against infantry heavy armies. The bonus that sword units have is NOT taken into account for autocalc.

[Arima]

???

They are always undervalued by the autocalc, as are all sword units against infantry heavy armies. The bonus that sword units have is NOT taken into account for autocalc.

Another reason why Autoresolve is unreliable.

[Cruzz]

Another reason why Autoresolve is unreliable.

It has issues, but it's still fairly reliable in sengoku and rots except in defensive castle and river fights which give no bonus to defender. Bow units are somewhat overvalued. Spear units are overvalued against sword units. Artillery units are overvalued now in their nerfed form, used to be undervalued. Cavalry are severely undervalued against armies without a lot of of spears, and overvalued against a large number of spears.

FOTS autocalc is pretty screwed up if you want to use traditional units though. For some reason CA gave rifles and artillery huge bonuses for autocalc strength, leading to a situation where facing off an equal number of shogitai against 0 vet line infantry actually make the line infantry win in the eyes of the autocalc. If you add one artillery to the line infantry, they don't really even lose any men at all. While if you fight the battle out, the shogitai will completely wipe the line infantry, depending on the map with low to very few losses.