The game's AI fails to recognize walls of any kind as a target, which can be exploited in a variety of ways. Apart from using cheap sandbag walls to protect your own base, you can wall off important passages, or even build a wall around the enemy base, locking his Harvester in or out as well as all of his units. You can then take your time to build up whatever army you need to clean up easily while the enemy can't do anything except watch from the other side of a wall it can't destroy.