Masons are commanders that can upgrade forts to one level higher than usual for their nation. For instance, MA Marignon can normally build forts up to Castles, but their Architects can go one step higher, to Citadels.

This is especially valuable for nations like this that are normally limited to Castles; with masons they can upgrade forts to Citadels, which give three commander points per turn rather than two. This is especially valuable for recruiting capital-only mages faster.