In StarCraft II, structures aren't just good for production. You can use them to create solid walls as part of a strategy known as "wall-in". Improvised walls (mostly used by terran and protoss) can prevent or repel early assaults and scouting attempts.
Rather than simply relying on defensive structures, like bunkers or photon cannons, which enemy units often bypass or destroy quickly, walling in takes advantage of existing infrastructure to bolster your defenses and make it more difficult for your opponent to attack your main base.
How, When and Where to Wall
Walling is most often used to hinder ground units without long range attacks, like melee units and scouts, and to negate rush armies from getting to your workers.
The best locations to wall-in are access points like chokepoints and ramps, where an opponent would need to send troops in order to effectively scout or attack.
To ensure that your wall defense is optimal, it's important to consider a few factors:
- The size of the units you expect to be attacked with. "Holes" in your wall can be nonexistent, small enough to allow only tiny units like zealots and SCVs to pass through, or big enough for an ultralisk. If your units can pass through or fire around your wall, but some of your opponent's units cannot, you will have a notable advantage. Think about the units your opponent might send to your base, and how would you like to stop or corral them once they arrive.
- The terrain close to your wall. Ramps and cliff faces are indestructible and of a fixed size, making them essential bricks in your wall. Rocks and other destructible elements can change in size or be removed from the battlefield, providing an area for your opponent to break through. Make them walls solid, son.
- The structures that will constitute your wall. Ideally, you want structures with large amounts of HP (hitpoints), as they will be able to withstand more punishment. Defensive structures, like bunkers or photon cannons, are not very resilient, but they'll deal damage to attacking units—more so if you place them carefully to take full advantage of the protection offered by the wall. Structures with tactical advantages (supply depots, which can be raised and lowered to act as gates, and barracks that can be lifted off) are the standard in terran wall construction.