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While watching a live-stream of a game (TvP, master league) I noticed that both players built their production facilities towards the entrance of their base, starting at their command-center/nexus. This was fairly early in the game, around the 5 minute mark.

Protoss building towards entrance

Terran building towards entrance

I also noticed this building-placement in some other games (except vs. Zerg where a wall-in is prefered) and wondered if there's an advantage in doing so?

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2 Answers

up vote 18 down vote accepted

So, I saw iNcontroL do this on his stream last night and I wish I had captured it for you because it really illustrates why this is such an important move: It controls the point of attack.

History

Back in Broodwars it was very common for your buildings to form a nice arc in front of your mineral line. The reason being that back in those days, every race had powerful melee and ranged units very early on. As a result, the arc allowed a player with ranged units to control where the opponent's melee units would attack in an artificial choke.

Starcraft 2 differs from Broodwars in a number of important ways: Smaller ramps at all naturals, mean you have a natural choke at every main base. Additionally, smaller bases mean that its easier to use buildings to create a wall to your natural choke, or from that choke to your natural expansion. This creates the line phenomena you observed, which has replaced the traditional arc

XvZ

The reason you see players do this largely against Zerg (and less so against other races) is because the balance of Ranged/Melee is now distinct amongst the races. Terran have a fairly ubiquitous ranged early army, where as Zerg's army tends to be almost always Melee (or short ranged) early on. Though some players would rather opt for a Wall-In, that has disadvantages, and often is not practical for expansions. As a result, it is often a better idea to build your buildings in a line (especially warp gates, where it doesn't matter where you put them).

Control

Finally, let's get to the reason that building these great walls is a good idea. If you have a range advantage on your opponent (or even if you both have a lot of short range), then if he attempts to bust your wall, you can harass him without taking damage to your army (your wall absorbs it instead). Most enemies will almost immediately realize why this is disadvantageous and withdraw. For them to attack you, they're forced to run around the wall (sometimes taking damage in the process), and engage at an artificial choke point. The Choke has long been known to give an advantage to the force with superior range, and since many natural expansions lack a choke, this artificial choke gives you a new advantage.

Summary

Whether or not you choose to wall-in, its important to know why you opponent might, and understand what its trying to accomplish. Usually the advantage provided by these walls starts to disappear in the mid-game, as players gain access to more ranged units and people start putting their army in front of their wall.


P.S.

Its worth noting that recently a number of Zerg players have shifted to what is called a high economy baneling bust. This is a move specifically targeted at these kinds of walls (and more general wall-ins). This utilizes an unusually high number of Banelings to break the wall, followed up by rushing Speedlings into the natural Mineral line. As a result, its important you keep your army at the part of the wall closest to the enemy, so you can catch this bust in action, and only move towards the choke when your enemy does.

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Thanks for the detailed answer. Creating an artificial choke at your natural seems like a great idea, but why do you think it's good for your main? You already have a choke formed by the ramp and usually you're on the high-ground as well. The reasoning by Wipqozn about protecting workers seems much more plausible? Also this was a TvP matchup, so the advantage of ranged units doesn't affect it as much? –  bummzack Apr 5 '11 at 16:54
    
@Bummzack, the natural choke is nice, but now imagine those Zealots running through the natural choke, along the wall, and around the artificial choke, while Marines are taking pot shots the whole time. Saying "it protects workers" isn't wrong, it just isn't descriptive. The method by which it protects workers is by controlling the point of attack. As for PvT, it is very common to send Zealot raids on mineral lines... –  tzenes Apr 5 '11 at 23:22
    
thanks for elaborating further. I'm going to accept your answer since it shows the "bigger picture". –  bummzack Apr 6 '11 at 7:16
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I can think of two reasons for this:

  1. An attempt to split up your opponents forces (some on both sides) so that you can easily overpower them.
  2. To make it easier to defend your workers. If your opponent storms your base, you can just put your workers on the side opposite they enter, giving your workers both more time to escape - as well as dodge the enemy.

A combination of the two is probably the main reason. It's defensive since it can protect your workers, but, if your opponent splits up their forces so you can't do that, you can simply overwhelm one half of the opponents army.

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I was also thinking along the lines of your first point but not about the second! You're right, it probably helps a lot when trying to protect workers. –  bummzack Apr 5 '11 at 12:48
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+1 for #2: Looks very much like an attempt to block a rush on the mineral line. –  Satanicpuppy Apr 5 '11 at 13:21
    
I really agree with #2. It seems like a very logical tactic. –  Khez Apr 5 '11 at 14:27
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