A barricade hinders movement of "enemy" ground units, forcing them to stop on the barricade's square, even if the barricade is unoccupied. However, if occupied, a unit in a barricade enjoy a defensive bonus which is double the bonus awarded by a fortress. A barricade can only be built on a square already containing a fortress. If pillaged, a barricade is "reduced" to a fortress. Workers can build barricades after the discovery of Construction.