All Units have a sequence of attack which depends on their position on the board. The priority for attacks is: Forward, Middle, Outside. Listed below is how the unit will attack when you place a Unit next to an enemy Unit.
Unit Placed in Front of an Enemy Unit:
Your Unit will attack what is in front of it, regardless of what is to either side of it.
Unit Placed Between Two Enemy Units:
If there is no target in front of it, your unit will attack the unit on the inside of the board. Your unit will attack enemy units on the outside tiles only if there is no enemy unit in front or on the inside.