Models can fight if they are in base contact with an enemy model when it is their chance to attack, even if the models' bases only touch at the corner.
Even models attacked in the side or rear may fight. In such cases the models are not actually turned to face their enemy – we simply assume that the individual warriors twist around as best they are able in the tight press of warriors.
Normally, a warrior can only strike blows against an enemy model in base contact. The most common exception is if he is making a supporting attack.