Suddenly, when the population reached 12, my builders began building another small hut. I thought this would progress them to 'master' status yet, despite them having learnt everything there is to know about building (i.e. the bar is 'maxed out') they're still only adept. If they can't learn anything else, how to I train them further? I can't dig for the treasure otherwise!

Thanks

Give it time. There are other opertunities for your builders out there.

You should have masters long before you need to dig for the treasure.

Also, it takes much more for a villager to go from "Adept" to "Master" than it does to go from "Trainee" to "Adept".

In between other puzzles that require builders, if they don't do it themselves, drop them on the huts. They'll fix them up and it will give them more experience.