The game atempts to reproduce this by having each army have a morale level. When routing or destroyed units reduce this to zero the game is over. This means that your army can have completely surrounded the enemy, have your cavalry rampaging in their rear area but still lose because your army gives up due to a skirmisher unit routing and taking you over the morale limit. The fact that the enemy are one point of their morale level and surrounded means nothing. When causing routs, alling units etc position of the enemy means nothing.
Position means nothing in any context except for hand to hand combat.
In ancient warfare position means everything. All of Hannabal's victories, and Scipio's probably, were the result of achieving a superior position. Whether it was a flanking force at Trebia, an ambush at Lake Trasmenie, defeat of the flanks at Cannae etc. In each of these the victory was not wone by arms but by position. The game does not allow you to do this.