Trinity:City Mechanics

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.

Content
Disaster
Economy
Influence on Others
Infrastructure
Outside Influence
Population
Resources
Leadership

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).
Harvest

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.

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
Population: Level x 10
Resources: Wealth x 5
Leadership: (Com1 + Awa1) x 2

1 Use the ability score, not the ability modifier.

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, then divided by 10, to find the max GP limit of a single item that can be purchased in the city.

Resources further impact currency, but does not affect max GP limit. 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.

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

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

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), and lost 76 points of Resources, bringing
it to -6. 

There is a 70 point difference between 70 and 0, which - divided by 5 - is 15 (so 15%); Resources are another 6 points below 0, so
that's another 6%, for a total of 21%.

Therefore, currency in Edge is worth 79% of its normal value.

To find the actual currency required to buy a 728 GP item, divide the normal value by the currency value (79%, or .79). This
results in 921.5 GP - so you need to spend 921 gold and 5 silver, to purchase a 728 GP item.

Another reasonable approach would be to multiply the Max GP Limit itself by the percentage, thus representing that more expensive items are unavailable due to a shortage of resources. Both approaches work; it's just a matter of figuring out which is better (perhaps even both would make sense, but that would require using a different ability for one of the two).