Usually, the benefits of a special feature only go to the army or unit that currently controls it. Control is calculated at the start of every phase, and it is possible for possession of the special feature to change during the course of a player's turn. To control a special feature, a unit of unit strength 5 or more must be within 12" of the special feature and closer than any enemy units that can control it. Units that are fleeing may not control a special feature.
Unless the players agree differently when setting it up, the special feature is treated as impassable terrain and blocks line of sight as normal.