As mentioned in the section for setting up the battlefield, there is a class of terrain called special features. This category includes several different items of scenery that can benefit the army in possession of them, and the struggle for control of a special feature may play a prominent role in the battle.
Such is the nature of special features that we recommend you only use one on the table. You can of course use more, but be prepared for the battle to be dominated by the fight for possession of these important terrain pieces.
A special feature is placed like any other piece of terrain. Each of the special features described contains examples of how that feature might be represented on the battlefield. A single piece of terrain might be representative of several special features so players should agree what sort of special feature a piece of scenery is before they begin placing terrain. If you really can't decide what something should be treated as, simply count it as a historic landmark.