The farm is not necessary initially. The Horde's job is to put pressure on the center of the map immediately. The bulk of an attacking force must hit the blacksmith within 15-30 seconds of the start of the game, and hit it with everything you have.

The reason for this is simple. Using the blacksmith as a starting point gives the Horde at least three points where they can block moves to the farm: two to the west, one to the east. This limits Alliance farm attacks to routes from the mine only, which, of the four possible paths, takes the longest.

With the blacksmith secured, the Horde can move to the lumber mill and farm and use the intersection off the blacksmith's southwest bridge as a start point for defense of all three.

Successful defense of this strategy requires the Horde to remain mobile, and that means not sitting on the blacksmith flag. They should be on or around that southwest intersection at all times unless moving to defend the farm, lumber mill or blacksmith.