It's not so much that it's not useful. It's just that the shield cost to make a new one is almost as much as infantry. Given the choice (access to rubber) most people would rather invest the shields in infantry. Useful? Yes. Worth the cost to upgrade Medieval Infantry and Longbowmen? Yes - unless you're strapped for cash. Worth building? Only if you don't have rubber, or you have a ridiculous production edge over the AI... or you have a specific reason for building it. Basically, Guerillas give you 2 things: 1) an option if you find yourself without rubber 2) an upgrade path for otherwise obsolete units. There are plenty of uses for Guerillas, but none are so essential or unique that it justifies building them over infantry (which defend better) artillery (which can bomard offensively as well as defensively) or Cavalry (which attack just as well and can retreat). This upgrade path for Swordsmen/MedInf or Archers/Longbowmen is the thing I like about Guerillas (and TOW Infantry). When playing vanilla CivIII, I would generally avoid building swordsmen or archers if I could get away with it - mainly because of the built-in early obsolesence. I would send hordes of horsemen against hoplites and legionnaires simply because I didn't want a bunch of obsolete swordsmen lying around when my horsement could be upgraded to knights and cav... In a way you could argue that the Guerilla affects ancient wars more than modern ones... If I were in a real tooth-and-nail battle, needing every unit I could get to the front lines as quickly as I could get them there... and a couple of cities could produce an infantry in 5 turns or a Guerilla in 4... I might consider building some. The Guerilla, like any unit or improvement, has its place and its use. Before building anything you ought to know why you're building it. Does that size 2 tundra city really need a marketplace or cathedral?