Forgive me if I just missed this somewhere, but how do you determine how many laborers are available in an area? The construction rules list the availability of skilled laborers and master craftsmen based on the the number of available laborers, but I don’t see the availability of laborers list in any of the ACKS books.
The sources of unskilled laborer are stated on p.30: “Unskilled laborers might be peasants, slaves, prisoners, conscripts, or even mercenaries on labor detail.”
In other words, it’s up to the leader to source his unskilled laborers anywhere he can! He can uses prisoners, buy slave laborers, etc.
To improve clarity: “When levying peasants to serve as laborers, apply the the same limits and effects as when levying peasants into a militia.”
I assume having dedicated craftsman/laborers/engineers in your army requires you to consistently pay their monthly, even when you don’t have a job for them. Do they require hazard pay for traveling with the army, even if they aren’t on the front line? Is their much precedent for having craftsman in an leader’s permanent employ for military actions?
The rules don't require that you pay them hazard pay, no. You certainly could do so if you wanted to.
Armies definitely retained craftsmen in permanent employ during the classical era, and sometimes during the medieval era.