City Mechanics

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I once randomly came across an interesting online sim, Heroes of Ivalice. Taking a look, I noticed that - in some combats - the city in which the combat took place was mentioned, and suffered adverse effects due to the destruction of property.

I immediately became intrigued. The idea of cities being mechanical constructs, that could be affected by the actions of players - and not just ad hoc'd, or by DM fiat, but with actual mechanics behind the idea! Being the mechanist that I am, I decided that I, too, wanted to implement something of this sort.

Unfortunately, d20 is sorely lacking in material. One has difficulty finding mass combat rules, much less rules for mechanical effects on cities as a result of a PC's actions. However, the space is not completely barren, in terms of published rules, and I have found something that will serve - for now - as a decent baseline.

Here, then, I will outline the base mechanics of cities, and how those mechanics interact with the core d20 System. Not all of this information is terribly relevant for players; for the most part, the DM should track this information behind the scenes. Astute players will figure out the more obvious effects (the power of currency changing, for instance), but others will pretty much be outside of their scope.

Building The City

A city is much like a character, in that it has levels (based on its population) and classes. Cities also have an analogue to Hit Dice, that being Wealth.

A city has six ability scores: Force, Mobility, Resilience, Learning, Awareness, and Command. Each of these six abilities correspond roughly to a character's ability (so Force corresponds to Strength; Learning to Intelligence; and so forth).

Cities purchase these ability scores using the point buy method. A city receives a number of ability points to expend based upon its population. In addition, its population determines its population level (ie, character level).

Once the abilities are determined, you can pick from one of three classes for each population level: civilian, military, and a class that indicates a particular force alignment. Each class contributes to the city's Defense, Reputation, Wealth, skills, and feats.

Of these, Wealth is the most important (and only fleshed-out one, at the moment). Feats are also important: feats represent city features that go above and beyond what is indicated by the class levels. For instance, defensive fortifications, universities, and factories fall into feats.

Once these are all calculated, a city then has a number of derived abilities. They are as follows.

Influence on Others
Outside Influence

Each of these works (theoretically) on a scale of 1-100.

In addition to those, the following may also be implemented (but are not, as of yet).

Once these abilities are determined, a city then has a number of link values, as each of these abilities is linked to one another. Events that take place in or around a city impact one or more of these abilities, which then further impacts the rest. Link values are assigned ad-hoc, dependant upon the nature of the city, on a scale of x(1/4) to x4.

A City's Event Flowchart

Whenever a change occurs in one attribute, that change is propagated throughout the chart. For a given event, when it travels down a link, each link strength is halved for each link previously traversed. However, an event can only affect a given attribute once, no matter how many times links would bring it there. The attribute's link still counts for purposes of continuing to weaken link strengths, however.

For the time being, the precise effects of the vast majority of these abilities is not determined. While they can be DM fiat'd into usefulness, I would much rather have solid mechanics for dealing with them. Later sections will detail how they function.

Calculating Attributes

The following formulas are used to calculate the initial values for each of the attributes. Note that these can change, and the system is designed to allow these values to fluctuate.

Whenever the city gains a level, revalue all attributes that are based upon an ability score that changed.

Content: Awa1 x 5
Disaster: 0
Economy: Com1 + Lrn1 + Rel1
Influence on Others: Reputation x 5
Infrastructure: Rel1 x 5
Outside Influence: 20 - Awa1
Population2: Level x 10
Resources: Wealth x 5
Leadership: (Com1 + Awa1) x 2

1 Use the ability score, not the ability modifier.
2 This is a simplified formula. Actual formula is... 
  (Level x 10) + Floor{10 x [(Actual Population - Minimum Population) / (Maximum Population - Minimum Population)]}

Wealth, Resources, and Currency

Rather than saying that a city of a certain population has a max GP limit of some number, the maximum GP limit of a city is determined based upon its abilities and classes. Wealth is determined by classes, which is then multiplied by population, to find the max GP limit of a single item that can be purchased in the city.

Resources, Economy, and Infrastructure each further impact currency. Instead, Resources modifies the value of 1 gold piece. If a city is at it's normal Resources value (Wealth x 5), gold pieces are exchanged at a 1:1 ratio.

  • Resources: Modifies Max GP Limit for the city, as a percentage.
  • Economy: Modifies the value of 1 GP, as a percentage.
  • Infrastructure: Modifies item availability.


Resource fluctuation causes the maximum GP limit of an item that can be purchased in a city to go higher and lower. For every point above the normal Resources value, the max GP limit goes up by 1%; for every point below, the max GP limit goes down by 1%.

In addition, if a city's Resources drops to 0 or below, the max GP limit drops 5% in value for each point of Resources below 0.


Economy fluctuation causes the currency to gain and lose value. For every 5 points above the normal Economy value, currency gains 1% in value; for every 5 points below, currency loses 1% of its value.

In addition, if a city's Economy drops to 0 or below, the currency drops 1% in value for each point of Economy below 0.

To determine Economy's effect on the price of a given item, divide the item's price by the currency value as a percentage. This will result in the city's price for the item.


Every standard item has an availability rating, on a scale of 1-100. Any item whose availability rating is equal to or below the city's Infrastructure can be purchased in the city at the city's price for the item (standard price modified by Economy).

If an item's value is below the max GP limit for the city, but its availability is higher than the city's Infrastructure, the item can still possibly be found, but at higher prices than normal. To do so, follow the following steps.

  1. Determine the Craft skill that would be able to produce the item. If the city has the skill, proceed to the next step.
  2. Multiply the city's skill bonus in the skill by 5.
  3. If the result is higher than or equal to the item's availability, the item can be found in the city.
  4. Subtract the city's Economy from the result in step (3). If the result is below 1, the item cannot be found in the city. If it is 1 or higher, subtract it (as a percentage) from the city's currency value.
  5. Divide the item's cost by the result in step (4) (as a percentage).
  6. The result in step (5) is the cost of the item in the city.

An Example

Edge has a Wealth of 14 and Population 52, meaning that it's Max GP Limit is 728 GP. 

However, Edge recently suffered an explosion in its industrial sector (+38 to Disaster), which resulted in a loss of 38 Content,
Infrastructure, and Resources; a loss of 19 in Leadership and Economy; and a loss of 9 to all other attributes.

Max Item GP: Resources is now 38 points lower, resulting in a 38% drop in Max GP. The new max GP Limit is 451.3 gp.

Currency Value: Economy is 19 points lower, to a 16; this results in a 3% drop in currency value. To purchase an item of the
max GP limit (451.3 gp), a person must spend 465.3 gp.

Item Availability: Infrastructure took a 38 point hit; Edge's original Infrastructure was 35, and is now 16. Items with an
item availability of 17 or higher are no longer available.

Let's now say that a character wants to purchase breastplate in Edge (35 availability).

Edge has the Craft (Blacksmith) skill, which is what is required to make breastplate. Breastplate's base GP value is 200 gp, lower
than Edge's current max GP limit. So the item can possibly be found, but will be (even) more expensive than normal.

Edge's Craft (Blacksmith) is +7, x 5 is 35. 35 is equal to or higher than the item's availability, so the item can be found.

Edge's Economy is 16. 35 - 16 = 19, which is greater than 0, so the item can be found. This is subtracted from the currency
value, resulting in currency's value being 22% of normal in regards to buying the breastplate.

To purchase the breastplate, currency is treated as 78% of its normal value, meaning that one will need 256.4 gp to purchase the


A "disaster" (as per the attribute) is any negative effect on the city that is not already covered by another attribute; that is, it represents natural disasters, negative outside effects on the city (such as an attack), and similar.

Disaster has an immediate effect on the city, namely on Resources, Content, and Infrastructure. These effects then propagate throughout the city, causing rather extensive damage throughout the system. More severe disasters may completely cripple if not outright destroy smaller cities.

After a disaster occurs, the Disaster attribute is gradually reduced over time, based upon the city's Command and Resilience. Disaster goes down at a rate equal to the city's Command modifier every day (therefore, a city with a negative Command modifier suffers ever-increasing effects from a disaster, due to poor leadership).

In addition to that, every (5 - Rel mod) days, the city can further reduce the damage from a Disaster, though this is more initiative on the part of citizens than leadership. As such, the effect requires what amounts to a Fortitude save: the city rolls a d20 + Rel mod + Defense, and consults the following table.

Save		Result
10 - 15		Negligible (1d4)
16 - 20		Minor (1d6+4)
21 - 25		Average (1d8+10)
26 - 30		Major (1d12+18)
  35+		Critical (1d20+30)

Any result above a 10 indicates that the city has had some measure of success in recovering from the disaster; reduce the Disaster attribute by the indicated amount. In addition, this recovery causes the populace to lose some amount of faith in its Leadership (as they are being ineffective, if the city has to resort to these extreme measures), and Leadership suffers a decrease equal to 1/4 of the decrease in Disaster.

Unlike normal increases and decreases, if the city makes a "Fortitude save" against a Disaster, the benefits of doing so do not propagate through the city at the same rate: halve the link strengths of all links from Disaster.