I usually build twin towers and man them, then get the extra arrow pretty early. Murder holes is next, then the extra reach when I can spare the metal. All depending who I am and what my situation is; sometimes, when there's no traffic at my borders (or I'm iber), I never build or upgrade stone towers myself and just do it later when I captured something. I hardly ever get the other two tecs, if I do it's extra arrow per garrison before (if ever) the extra health.