There are at least three guides to trade in Oriental Empires. This monograph is intended, instead, to provide hard numbers and essential rules for this poorly documented and indifferently understood part of this game. Only a small amount of direct advice is given, directed towards the most commonly-asked questions.

Game version: October 7th, 2020 update, no scenario or added rules
Guide Version: March 7th, 2021

You may expect some errata in this draft version of the guide; comments and corrections are most welcome.

So. Trade. Trade starts off small, but - if you nurture and protect it - snowballs into so important a source of revenue that it may yield more than twice as much as everything else combined. And trade does so without making your peasants salty about the Salt Tax.

Therefore, as the game proceeds, you will want to:
a) expand your trade network,
b) provide new goods to be traded on it, and
c) stack up multipliers and bonuses, ...
d) while being mindful of maintenance and corruption losses.

Trade all about making a profit: Earning revenue that exceeds both building maintenance and corruption losses by enough to make it worth paying the up-front cost of buildings. You will need to make careful decisions about when - and whether - to invest in your trade system.

You collect taxes on each good traded, for each destination city. Most goods and all trade routes are generated by buildings.

Trade value is based on the innate value of the trade good, the destination city's population, the multiplier to trade granted by the trade building generating the trade route, technology, location, faction attributes, and other factors (some discussed below). Value is added per population unit in each city: at full rate for the first 40, at roughly half rate above 40 (therefore: a population of 60 is effectively 40 + (~1/2 * 20) ~= 50).

Goods are sold within cities (Internal Trade) and between cities (External Trade). Goods are produced in an internal building (eg. a Bazaar or a Pottery Shop) or an external building (eg. a Bronze Pit or a Quay). Goods are always sold within the city that produces them. If the city has (at least) a Quay, then they are also shipped to other cities with Quays in range. Otherwise, if the city has at least a Bazaar, then they are also shipped to all nearby cities.

Cities that produce their own goods have no, or much-reduced, demand for that good from other cities. Therefore, for each good, you will want one city to supply many.

Trade has a limited range, and always plots from city center to city center. Waterborne trade plots from a city center to a Quay, along a (navigable) river or across a sea to another Quay, and finally to the destination city. Landborne trade plots directly from city center to city center, including across 1-tile rivers, but cannot cross harsh terrain. You may show and hide trade routes by pressing Alt-T.

[1] This good is considered a luxury item, and adds seven to noble happiness.

[2] The "Base Value" provided here is base revenue for the first 40 population in each destination city. The base value for all additional population is roughly half of this. The Base Value:
a) is approximate and may even be incorrect; corrections are welcome,
b) assumes a trade multiplier of 1 (you will almost always get a multiple of this; Bazaars grant x2),
c) is with bronze given a bonus of 50% by early techs (you will likely have these techs by the time you can set up a serious bronze trade network), and
d) has no other bonuses or penalties applied.

[3] The maximum multiplier due to building level. For example, while Bamboo cannot be upgraded, copper mines can.

[4] Iron replaces bronze. While this cancels the existing bronze bonuses, iron eventually gets higher ones.

Note: All trade buildings other than the Caravansary produce the Local Specialties good.

[1] Land-borne transport plots from city center to city center. It cannot cross forests, steep hills, or mountains. It can cross water. Roads that connect cities add a 20% bonus. The other city does not need any building to be traded with.

[2] Water-borne transport plots from city center to quay, along navigatible water hexes to another city's quay, and finally to that city's center. Range includes all steps. The other city must have at least a Quay.

[3] The Build At value for trade-generating buildings can only be a fuzzy number. Firstly, it ignores the fact that increased range is often important. The Value listed is population for the lowest-level trade buildings and existing trade value (internal and external) for upgraded buildings. For the lowest-level buildings, it is assumed that only a trade building and a Pottery Workshop will be built (because all other production building require a bit more population to become profitable than these do), and that no special resources are present. For the Quay, a network of three Quays is assumed. All values are approximate.

[4] The Caravansary building resets trade range, but halves the trade multiplier of any re-exported goods. It appears also to cancel any multiplier from advanced mines, so a Deep Jade Mine yields the same value as a Jade Scrape when re-exported.

[1] The Build At value is the total population that goods produced by this building needs to reach in order for it to be worth building, or upgrading.
This is a fuzzy number (although not as fuzzy as that for trade buildings) because a bunch of factors come into play. Here, we assume we have only a basic Bazaar (x2 value multiplier), all destination cities have a population of <= 40, we have no special bonuses, we want to earn the initial investment back in 30 turns, have only Early Bronze Age tech, and lose about 20% of added income to corruption (a not-unreasonable guess for a faction in the late Early Bronze or early Warring States period). For empires with more bonuses, a more well-developed trade route, or a rapidly increasing population, payback is quicker, and you can be more aggressive in upgrading goods production. For factions with higher corruption, payback is slower.

[2] Mountain must be in city range + three hexes. For a small town with no expansion yet (two hex range), it must be no more than five hexes away (six, if you can expand the borders with a farm). Not all mountains allow the building of a Shrine; others allow the building of Monasteries, which have no impact on trade.

[3] The Break-even population for the Iron Foundry drops drastically late in the game with advanced iron technologies.

• Trade values are rounded down after calculations.
  For each population unit at or below 40, a Bazaar yields an x2 multiplier for external trade and x1.5 for internal trade.
  For each population unit above 40, a Bazaar yields a roughly x1.1 multiplier for both external and internal trade. No, I don't know why the multiplier is not simply x1.
  
  
• If a city is importing any good from more than one city, each exporting city gets an even share of the base yield. Each exporting city multiplies this value by any applicable multipliers, bonuses, or penalties.
  
• As stated in the game manual, empires with superior Culture get a 50% bonus to trade value, and with inferior Culture a 50% penalty. The bonus applies to any goods exported. The penalty applies only to goods competing with those exported by an empire with superior Culture. If you have inferior culture, but are exporting Falcons to a city with no other source, you get full value.