What three things were Han soldiers responsible for when they were manning the Great Wall?

Our Chinese infantrymen are at their best in obstructing strategic passes, while the barbarian cavalrymen are at their best on the flatlands. Let us resolutely stand on guard (at the strategic passes) and not dash off in pursuit of them or strive to chase them off. If they come, we should close strategic passes so that they cannot enter; if they withdraw, we should close strategic passes so that they cannot return.”