The option is just not there. Workers can build canals alright.

IIRC, slaves are generally unable to build unique improvements.

Thanks a lot for your answer!
This is only my third start in RI, so I didn't have a chance to discover stuff on my own. I've read through the manual and wish it would highlight more of the game mechanics such as double the great general points inside cultural borders and changes made from vanilla for the sake of those joining in at this final stage of the mod's development. The fact that I can't build cottages on fertile soils came as a total surprise to me as well and forced me to abandon a game (I don't like reloading) since the plan to build cottages on those tiles affected my city placement from the very beginning.

There is one more thing that might be or not be a bug with equal probability:

Is the bonus from civilizations' UBs supposed to stay in their cities after they switch hands? In my first game, the bonus from Rome's castrums (+1 food to granary and +1 hammer to forge) stayed in their cities even if all the relevant buildings were destroyed during conquest.

Thanks again for this beautiful piece of art!

If it stayed after a building got destroyed, then this is definitely a bug (though at this point I am not sure if I will ever be able to fix it).

If you have more particular suggestions of what to include in the manual, I am all ears - I do hope people read it and I tried making it a useful primer for new players.

It is not the most annoying bug as it adds flavour not only to playing but also conquering every new civilization.

As to what would be helpful to read in the manual:

- The experience cap from fighting barbarians is now 25 (? I just reached 25 now with a unit, so as far as I know, it can be even higher). I still don't know what's the cap from fighting animals.

- Fights against barbarians give GG points. Slave rebels can't pillage. Related to this: do all cities have an equal chance to get a slave/serf rebellion? I noticed a heavy tendency of rebellions to be more frequent around bigger cities with slave markets. I don't think I got a single rebellion around a newly-founded city so far.

- Experience gained inside cultural borders contributes to GG emergence twice as much as that gained outside of them before the protective/Great Wall bonuses are applied.

- The mechanics of epidemics. I'm lucky enough not to get a single epidemic so far while the AI around me often has it pretty bad. Thus, I don't know how much having an epidemic in one or a couple of my cities would affect other cities (if all the trade routes are foreign, for example), or what are an epidemic's chances of causing specific damage: killing 1 pop, 3 pop or 5 pop, say. I can't evaluate how viable or risky it might be to grow cities out of their epidemics' safe zones.

- The two things that have just been mentioned if only to avoid that first-time disappointment: no cottages on fertile soils until fertilizers and slaves being unable to build UIs.

- I keep seeing, normally besieging, troops spreading their culture around cities. There is no mention of this mechanic in the manual, whereas I haven't yet come across the mechanic that is mentioned there: "Additionally, if your relations with another civ are high, you will get bonus culture from having open borders, as your peoples exchange their cultural trends with each other." I haven't been on friendly terms with civs that often so far however.

- Minor mistakes in the manual: Tobacco is mentioned as a resource that provides a happiness bonus instantaneously. It doesn't, hemp and dye do this instead. It says that one coal can "power" a brick factory, whereas you need 2 coals for it. A presumably outdated Guild Monopoly civic is mentioned. Ulugh-Begh is incorrectly mentioned as a Mongolian leader. Tech transfer bonuses are specified as 50% & 25% rather than 40% and 20%. A typo with zeroes here: "When the total number of alive and independent (non-vassal) players drops below half of the initial number, and one player is at least 50% ahead of everyone else in score, the “End of History” modifier kicks in, reducing separatism by -100% per each fewer civ below the threshold (capped at -90%)."

- There's a mix-up in the description of light infantry: recon units don't consume food during construction and irregulars don't normally posses any particular mobility or withdrawal abilities.

- The manual incorrectly specifies the strength ratios that are required for the three levels of aid. It specifies the first tier to be 100-200%, but a str 3 archer provides first level ranged aid to str 4 units.

I think this covers most of the significant issues I've come across so far.

The civilopedia entry for Canal can be made more informative my mentioning that it carries irrigation with irrigation systems and not right away and that it can only be built by a fresh water source, i.e. canals can't be chained.

Again, IIRC, it can be chained when it carries irrigation. And everything that potentially can do so starts to carry irrigation with a single tech, that's an engine limitation.

It makes total sense, but Irrigation Systems is not mentioned in the Canal entry and Canals are not mentioned in the Irrigation Systems entry.

I'm attaching a screenshot: the selected worker on the right will build an irrigated farm but won't build another canal. The same applies to the worker by the left canal. A bigger image: https://drive.google.com/file/d/1d0l_bH ... sp=sharing