I've been told that it's a waste of time, but . . .
Often in the period between the consumation of road-building and the acquisition of Rep tech, I found that there were a lot of workers with nothing to do. Roads everywhere they're needed and every tile improved to my satisfaction.
So rather than simply sit them down for a long lunch break (they do this if you zoom in on the graphic), I send them around my borders building fortresses. Pick likely locations, hills, forests, behind rivers, etc. Space them 2 or 3 tiles apart, and put a longbow in each one--upgrade them to rifles when the opportunity arises.
In my experience, it hasn't been a waste at all. When the inevitable DoW comes from Shaka/Monty/Alex/Ragnar, the fortress units warn me where the strike is aimed. Sometimes, the forts fall--too bad for the single unit manning them. But on other occasions, the centrally-located fire brigade of mounted units can actually reach the fortress in one turn, foot armies in two or three. This has resulted in fighting an enemy stack from a fortress on a hill/forest/forested hill/behind a river. It also happened that, once the counter attack began, and my forces starting pushing into hostile territory, seizing enemy cities, I could add the fortress troops to either the mobile field armies or use them as additional occupation units. The forts also can be positioned over resources, thus adding to the overall trade complement, and--a fort as part of a line is mutually supporting; there won't be a lone fort out there on that oil/silver/fur/whatever waiting to be picked off.
FWIW.