Patronus,
Squalor is caused by population. One point of squalor for every 3000 population, to be exact.
Every point of squalor gives you -1% to population growth.
Every point of squalor gives you -10 to PO.
To control squalor, you must control population. TO control population you must either control population growth or eventually end up killing people (through rebellion or disease). The easiest way to control population growth is to very carefully manage your farms (i.e., do not build farm upgrades past land clearance, unless you know what you are doing).
For some preventative and curative actions you can take to counter squalor and low P.O., please see a post I wrote addressing this. I'll see if I can find it... There are also some good posts by others, if you search for them, as this question comes up a lot.
EDIT: Found the post. Here is a reprint (edited to make it more readable) :
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Originally Posted by
npoe
... Do not build farms; they are bad for controlling population.
... There is no easy way of control squalor...
Controlling farm upgrades is indeed one of several ways to hold population in check.
There *is* an excellent way to control squalor: Control population! (Which means control PG.) The effects of squalor are twofold:
1.) Effect of squalor on PG is --> PG (due to squalor) = (-1%) *(population/3000). So a population of 3000 generates -1% PG, which offsets the positive PG that you get from buildings, etc. A population (POP) of 6000 generates -2% PG, and so on. A HUGE City of 24000 generates a PG due to squalor of -8%.
2.) Effect of squalor on Public Order (PO) --> PO (due to squalor) = (-10) * (POP/3000).
Your PG and PO due to squalor are exactly defined by the total Population (POP), as shown in the equations above. After many turns, your PG due to squalor will exactly offset "non-squalor" PG. For example: If your city buildings and other causes add up to a PG of 6, then your city will grow until it achieves a -6 PG due to squalor. Therefore, in the long run (after many turns) your PG and PO due to squalor are exactly defined by the total ""non-squalor" PG that you build into your city. This "non-squalor" number includes the PG effects of farms, most market upgrades, other buildings, tax rate, and governor's attributes/retinue). Population will continue to rise until the negative PG due to squalor exactly offsets the positive PG that you designed.
The negative PO due to squalor is exactly ten times the PG due to squalor, which is the same magnitude as ten times the PG you designed into the city. This relationship is shown in the table below:
(Designed PG = DPG, PG due to squalor = SPG, PO due to squalor = SPO, population = POP)
Code:
DPG SPG SPO POP Comments
1.0 -1.0 -10 3000 Too small
2.0 -2.0 -20 6000 ~7900 POP is good taxes per person.
4.0 -4.0 -40 12000 Large City (takes forever on Normal tax)
4.5 -4.5 -45 13500 Get a large city faster
5.0 -5.0 -50 15000 Get a large city even faster
6.0 -6.0 -60 18000 Little advantage, if any, over large city
7.0 -7.0 -70 21000 Little advantage, if any, over large city
8.0 -8.0 -80 24000 Huge City (takes forever on Normal tax)
8.5 -8.5 -85 25500 Get a Huge City faster, but PO problems
9.0 -9.0 -90 27000 Get a Huge City even faster, but PO problems
10. -10. -100 30000 Too big!!
11. -11. -100 33000 Too big!! (PO penalty due to squalor maxes out at -100)
So, pick your PG to design your final POP, which locks in your final PO penalty due to squalor (SPO).
For "civilized" factions: You probably want to design most of your settlements to reach 12000 to 15000. This gives all benefits except the very top tier of buildings; and the PO penalty (due to squalor) of -40 to -50 is easily overcome. A few of your cities you will want to be HUGE Cities. It is best if these are your starting cities and/or close to your capital; so that you don't also have to deal with conquest, cross-culture and distance-to-capital penalties.
For "Barbarian" factions: You probably want to design most of your settlements to reach 9000 to 12000. This gives quick growth through the 6000 population level, which gives all benefits of the top tier of Barbarian buildings. The extra population above the needed 6000 gives a little more cash; and the PO penalty (due to squalor) of -30 to -40 is very easily overcome.
In addition to the PO due to squalor, remember that PO can be negatively affected by tax rate, governor attributes/retinue, recent conquest, cross-culture penalty, number of troops stationed in town, distance to capital, presence of mercenaries (I think), enemy spies sowing discord (I think), damaged buildings (I think), lack of infrastructure (I think), and probably other factors of which I am unaware.
The biggies that many do not think about is the conquest penalty, the cross-culture penalty, distance to capital, enemy spies, and infrastructure.
The conquest penalty is something like -50 to -60 PO, max, and drops off gradually after conquest.
The cross-culture penalty is also around -50 to -60 PO, max, and is a function of how many buidings in the town do NOT reflect the culture/religion of the town.
The distance to capital penalty is 0 within 15 squares, maxes out at -80% for 86 squares and varies roughly linearly with straightline distance between 15 and 86 squares distance. Therother did some excellent research on the subject and even posted a coordinate map of all the cities in RTW in this thread.
I do not know how to quantify spy and infrastructure penalties, and can't even prove they exist; but I *HIGHLY* suspect that enemy spies and stunted economic development (lack of roads/ports/markets) contribute to negative PO. I do know, for a fact, that when an enemy spy enters one of my towns, that town's PO almost always drops like a rock.
There are exceptions (like, I think, if you have bribed generals working for you) but typically the culture of a town is YOUR faction's culture. Reduce cross-culture penalty by upgrading or incrementally ripping down and replacing the offending culture's buildings. I think (but am not sure) that the governor's mansion and religious shrine/temple have a disproportionally large effect on cross-culture penalty. Be careful not to rip down a temple if PO is already marginal. Keep a spy in town to root out enemy spies.
Typically, you can keep population and PO under control by controlling tax rate, buildings, etc. However, if you are, for example, playing the Gauls and conquer a city with the top Greek farm upgrade (which can neither be destoyed nor upgraded) it could be tough, even if you started out by exterminating the city.
Remember: If you try to control PO by lowering taxes, you are only pushing the problem out a few turns and making it worse. This is because lowering the tax rate increases PG, which results in population growth, and more squalor to deal with later. Lowering taxes should only be used as an emergency SHORT TERM fix, while you immediately take other measures to allow you to raise the tax rate again.
Bottom line: If you are having widespread population/PO problems with settlements you have controlled for a long time, then you almost certainly haven't been paying attention to the long term PG effects of buildings/upgrades (especially farms).
Prevention (especially not upgrading farms) is a lot less painful than the cure. But since you are asking the question, you are probably past the prevention stages for many of your settlements. So-o-o-o, here is some "Post-prevention" advice: The cure can entail:
1.) Stationing a governor in town. PO bonus for a governor is +5 times the number of green wreaths (influence) he has. (Example: 4 wreaths = +20 to PO.) There is a -15 PO penalty if you have niether governor nor troops stationed in the settlement.
2.) Stationing troops in town. There is a cap of +80 PO bonus, for troops = 12% of POP. Example: For a town of 24000 you would get the max troop bonus by having 24000 * 0.12 = 2880 troops stationed in town. If unit size = 40 men (normal units) then each unit is worth about (80 / 2880) * 40 = +1.11 PO. Large units (80 men) give +2.22 PO each. The next step up (160 man units) is +4.44 PO per unit. And so on. My understanding is that the PO bonus for peasant units is half. (EDIT: I have since heard that the derated effectiveness of Peasants is true only for BI. So that makes the large, cheap Peasant units by far the most cost effective way of keeping order in vanilla RTW.) So, on "large units" setting, the max PO bonus for troops would be 2.22 * 20 = 44.4 = ~about~ +45 PO; and remember that this is uses up all 20 slots, so no governor. On the next unit size setting up (forgot name) 18 units (of 160 men each) gives max bonus of +80 PO, and leaves empty slots for the governor and another unit.
3.) Eliminating cross-culture buildings. I forget the exact formula for Cross-Culture Penalty (CCP), but think it is something like this: CCP = (-50 PO) * (number of foriegn culture buildings) / (total number of buildings). I think the governor's manison and the religious temples are weighted more heavily than a normal building.
4.) Eliminating conquest penalty. Not much to do here, except ride it out. This penalty can start as high as -50 or -60 PO, and reduces by itself over time, and is probably not your main problem.
5.) Reduce infrastructure penalty. Make sure you build/upgrade roads, ports and markets. Remember that market upgrades also add to PG, which eventually leads to increased squalor penalty. Continuing this line of thought, I would **suspect** that a lack of trading partners could contribute to PO penalty. Sorry, don't know any formulas.
6.) Pick temples that give max PO bonus (as needed).
7.) Building public health buildings might help. I honestly don't know what these do, besides keeping plague at bay; and I don't even know how to quantify that. Some say that they give a POP boost with a corresponding PO bonus to cancel out the effects of squalor on PO. If so, then it is like getting free population with no PO penaly. If not, well, there might still be an indirect PO bonus hidden in there somewhere.
8.) Station a high subterfuge spy in town, to ferret out any enemy spies and assassins. Have an assassin handy (in town or very nearby) for the bonus of getting a kill on the weasel that did this to you (and therefore preventing him from repeating the performance). Again, sorry, no quantitative formulas for how much damage an enemy spy or assassin can do. I've been told (but have not confirmed) that if a building is damaged, even in the slightest, that for some buildings, you can lose the entire benefit of that building. If true, then a spy/ assassin team can really shaft you if they go after an advanced shrine.
9.) Remove mercenaries from town. I have not heard others on this subject, but in my own games, it appears that moving mercenaries into some towns decreases PO. Again, no quantitative formulas.
10.) Give yourself a head start by exterminating large settlements when you capture them. Enslaving can not only -NOT- give you the breathing space you need to get this town on its feet; but can also aggravate problems with your other cities that already have POP problems, that are now getting a POP boost from the slaves coming from this new settlement.
11.) Move your capital. If your inner cities are doing well (and they should be, since you have owned them from the beginning of the game, and you planned them well, right?) then you can move your capital closer to the center of your conquered territories to reduce the distance penalty there.
12.) The technique I always seem to see offered first (which, if you have to use it on your core cites, is proof that you have not mastered city planning 101), is to move your entire army outside of town and set taxes to very high. The idea is to drive PO as low as possible. When the city rebels, recapture it and pick the "Exterminate" option, to cut your population to 1/4 of it's original size.
13.) Another sneaky dirty trick of last resort is to move troops, or agents from a city with plague, to the overcrowded city. The overcrowded city gets the plague, reducing population "naturally."
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EXAMPLE:
Let's design a "cash cow" Roman settlement from scratch: We want a happy (PO = 100+), healthy, easily managed (even with the governor away), and heavily taxed, Large city (POP > 12000).
Pick POP = 13500, to get us past the 12000 mark quickly. This means "PG due to squalor" will eventually be (-1%) * (13500/3000) = -4.5%. We must design a +4.5% "Non-squalor" PG into the city. So, EVERY PG effect except "squalor effects" must add up to +4.5%. We want VERY HIGH tax, which adds in -1% PG. Assume no governor, or a governor that has no traits that affect PG. Then we must design the city to have buildings that add up to +5.5 PG (+5.5% from buildings - 1% penalty for VERY HIGH tax = the target +4.5% "non-squalor" PG).
When your town is small (say POP = 3000) momentarily set taxes to what you want in your final city (VERY HIGH tax, in this case) and write down the PG number. Lets say you write down that PG = 2%. Remember that this is total PG. We are experiencing -1% PG, due to POP=3000, which means our total "non-squalor" PG in this little town is currently 3% (non-squalor of 3% - 1% for squalor = the 2% you read in the status screen and wrote down).
So, current "non-squalor" PG is 3%, and the target final "non-squalor" PG is 4.5%. That means you can only add in another 1.5% PG worth of buildings. Go into the building browser and choose the desired combination of FINAL market/temple/farm upgrades to add in, to get that additional 1.5% of PG. Remember to only add in the DIFFERENCE between the PG bonus of the final buildings and the ones you already own. You want "great forum," and either "land clearance" or "communal farming," (which affect PG) as well as the maximum obtainable road and port and mine upgrades (which do NOT affect PG).
If you don't use up your 1.5% with the farm/market items, then you can afford the luxury of adding in farm upgrades. Add just enough farming to make the final number of extra PG bonus = 1.5%. When the town is complete, per this plan, it will have a non-squalor PG = 4.5%, and PG due to squalor" of -4.5%. This will give you a PO penalty of -45 "due to squalor."
Lets say you also get -40 due to VERY HIGH taxes (I think this varies with distance to the capital), for a total PO penalty of -85. To offset this, choose the awesome temple of Jupiter that gives +40 bonus to PO, and build an ampitheatre to collect another 10% and hold monthly games to collect still another 20%. That leaves 15% penalty. Since you probably want a little protection from intruders, anyway, train up 4 units of Peasants. On "Large unit" setting, and POP = 13500, this gives you a 15% garrison bonus; at a cost of 400 denarii per turn. This garrison should mostly be offset just by being able to tax at VERY HIGH instead of just HIGH.
Voila! You are at 100% PO. If you are a cheapskate, like me, forego either the garrison (if your city is very well protected) or the monthly games, and run at a PO of 80% to 85%. Remember that with no garrison and no governor: in addition to no garrison bonus, you run a -15% PO penalty for no military presence. I typically keep the garrison and forego the monthly games. The nice thing about this setup is that if you get into PO trouble from an unexpected source (e.g., enemy spy action), you typically have three (3) options to immediately boost PO: 1) run more frequent games; 2) move in a governor with influence; 3) lower taxes.
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Hope this helps. (And anyone who absolutely positively knows any formulas that I am missing, please chime in!)
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squalor
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I agree with Paul on the migrant thingy though.
. after some time, a city will negate to the orange status, which means 0% population growth, and no more increasing squalor. if you're lucky(assuming you already have maximum level government building), it will lower into red status and start reducing population. your goal is to reach this peaceful point without having the city revolt.
)
Actually, it mostly depends on version/mod one is playing. In vanilla, reducing growth with taxes works fine 'cause the growth rate is basically still quite high, no matter what you do. In Chivalry:TW mod, on the other hand, it's a suicide 
