When you approach an enemy or when he approaches you, make it a point to keep your army on a hilltop or at the foot of a mountain or on the side of a river so that you easily watch the movements of your enemy. Do not involve the whole army in the encounter, allow only a few units to take part in the engagements. If your army is not on a hilltop then post your scouts and guards on high vantage points and along the line of fortification so that the enemy may not take you unawares.