When a stack passed it's command test it will move to an adjacent tile. If the stack has not moved before in that player's round, the stack can enter any adjacent tile that is connected by a road which arrows point towards it. If the stack has already moved that round, the stack has to follow the arrows in the road (if you start anywhere where the arrows enter the road (the arrows point towards the center of the tile) and go on in the direction arrow for arrow you will leave the tile always at the same edge).
I decided to restrict the direction of movement to give a reason for planning ahead and making decisions. Also it would have been too easy to go to valuable tiles, if the players had complete control over the direction of movement. Together with the rule that you have to move at least one stack (passing a command test) this often leads to battles a player would have preferred to avoid.