Pretty commonly known but always fun. The AI is not very keen on destroying your walls, even the poor little sand bags. As long as you don't have any units nearby on the other side of the wall to tempt him, it will be impregnable.

Any sort of building can be built next to a wall, origionally intended for the creation of complete circuits of walls without having to closely shadow your base. Using the the strat formerly mentioned, one can build a line straight for an AI's base. Most effective in base leaching strategies and building forward production facillities.