A trainer should be created before soldiers

A rather simple solution would be to give each tribe two trainers in the starting conditions. In bug 669699, where the buildcosts of trainers have been discussed, no tribe was given additional weapons. This change would therefore re-establish the state before the change, where a player could produce trainers easily.

This would only solve the issue for the first two training sites, but that should be sufficient IMHO.

I also think it would be a good idea to at least give them at least 1 trainer at the start - this can be done very easily in the starting conditions scripts.

We should still have a look at the economy demand for a trainer though before we build a soldier - this will have to be coded in C++.

i also believe the making of trainers should have priority over soldiers. you normally need only a trainer or two, while you need dozens of soldiers. and in general, startiing to train soldiers is better than having one more level 0 soldier.
giving two starting trainers would not fix the problem: one may want to make a second training site as his economy grows (1 colosseum + 2 training sites = 3 trainers needed), and it wiilll never get filled as every single set of spear and helmet would become a soldier that is sent to the colosseum.

This point will become moot when we implement https://bugs.launchpad.net/widelands/+bug/1075562

Allow trainers to be recruited either in warehouses/headquarters or in any training facility or other building that consumes the tool/weapon. So any old carrier will do as an input (along with the weapons in question). Also clearly showing a dialogue of what ware is needed to make the trainer (and where) would be appreciated in place of simply stating "(vacant)".

In economy config, I assume that you can already specify create extra weapons beyond those needed?

Related tangent: in empire campaign mission, it asks you to set economy target of 50 wheat reserve, but there's no way to actually do that, as it's a base-level good (like water, timber) that is auto produced even when not strictly needed. You can still meet the 50 minimum requested and it triggers the next mission goal appropriately...it's just that the player can't actually control the economy in the fashion indicated. Sorry for hijacking a bug thread with this unrelated tidbit.

The Empire scenario gives instructions on how to configure the economy window to get the 50 wheat - if the scenario instructions are not clear to you, can you please open a new bug?

We decided not to train trainers in the Barracks - see the following bug for the discussion: https://bugs.launchpad.net/widelands/+bug/1075562

I have attached a screenshot that show how to access the information about what is needed for a trainer. Click on the ? on the bottom right to access the help menu. The Barbarian Trainer needs an Ax, so the next thing you should to is click on the Wares tab and check out what you need to construct an ax.

Fixed in build20-rc1