Entering the Third Century, the Roman Empire, under Hadrian, executed its grand strategy based on the simple concept of perimeter defense. This perimeter defense consisted of legions stationed within fortresses on the Roman frontier, and some were even accompanied by large stone walls (the most famous being that of Hadrian's Wall in North England). Another factor that made it easy for the Romans to adopt this type of perimeter defense strategy was the reputation of it's army, that is, the Roman army was so tactically superior to it's enemies, that it was in itself the biggest deterrent to would-be attackers, not necessarily the fortresses and walls placed along the perimeter.
Another advantage of this strategy was the low cost of army/troop maintenance; in fact, the Roman Empire at one point defended an empire of 50,000,000 people with an army of merely 300,000. This low cost and small army was directly related to the fact that Rome did not employ a central reserve in order to protect the internal empire should the outside perimeter collapse.
It was instead based on a defensive system of networked roads and rivers, through which, the small special units tasked with interior defense could quickly travel to reinforce a troubled area. Thus, all available manpower could be brought forth along the main line of battle and this further enhanced the fighting spirit of the troops already engaged on the front line--they knew that they had a strong and dedicated army coming to reinforce them, and they fought harder because of it.
However, despite that at the time this was a tried and true strategy and use of military manpower, it must be understood that a grand strategy of a nation consists of more than just it's military--but also it's politics, diplomacy, economics and, sometimes, religion.