It's not really critical, as Gatling Guns can't shoot over walls and they're outclasses by archers anyway, in range and damage when you consider the troop space. Guerillas are niche troops, insofar as they're good on paper but it's kind of hard to stay away from defenders so you have to get them backup. More trouble then they're worth versus offensive power, IMHO.
I built and capped it early, like the third new structure once EA. Refinery was my 1st task then. My main purpose of wanting the factory was it's troop space, 2nd purpose was it was another time consuming building attackers need to destroy in war.