I don't really know what system the German tribes or Caledonians used. NOr the steppe warriors of the Ukraïne and such. I wager it was mostly a range of watch towers, spies, small outposts and messengers to warn of any impending threat. Most of them were far too small to wield a standing army. The Sassanid empire, or Parthians, or whatever ruler of Mesopotamia at that time, did have the manpower to wield such a force and thus I think they might have a system of small forts, but most likely most of them would've been spies and watch towers. I don't think there were any equivalents of the limes or walls (such as Hadrian's) at Rome's neighbours.