Siege tanks love some high ground.
With regard to narrow passes, if you can occupy them first, let them be strongly garrisoned and await the advent of the enemy.* Should the army forestall you in occupying a pass, do not go after him if the pass is fully garrisoned, but only if it is weakly garrisoned.
It can be difficult to learn the ins and outs of every map without quite a bit of time spent out of game or ranked matches looking up every nuance of each one. I’m certainly not going to suggest you do that. Instead, I’d like to touch on some general suggestions and observations when it comes to terrain.
A lot of build orders call for you to send a worker to scout. You may see the pros doing it too. The truth is, other than giving you the initial position of the enemy, this potential loss of an early gatherer does not provide more vital intelligence, as to what your opponent is currently building or when their attack is coming. Likewise, unless they invest in some other means of observing you, they won’t have that intelligence either. Most of these terrain suggestions will provide you with some means to gather that intelligence, and possibly exploit the advantages some of your units provide.
The Higher Ground
StarCraft 2 maps are not flat surfaces. Plateaus, cliffs and ridges run throughout the battlefield. Many historical military engagements have proven that significant advantage exists in positions upon higher ground. In the case of StarCraft 2, if nothing else, you have the capability to look down into a trench or pathway and see where the enemy is going and in what numbers – at least until you’re spotted and they turn your brave scout into a bloody smear.
The high ground provides opportunities beyond mere observation. Certain units – seige tanks and colossi for example – lay down their fire in an indirect fashion, spreading destruction over a wide area. Doing so from a higher position limits the amount of retribution that can be brought against that position. Such planning can stall or possibly even stop an incoming attack, buying you more time to mount your counter-offensive.
The Path Less Traveled
In most cases, a player with some skill will quickly outgrow their initial placement and need to expand. There are some builds that can take you entirely to success on one base, but sooner or later you’re going to run out of minerals. Expansions solve the potential problem of economic shortfall, but these positions are more exposed to entry, especially from unexpected angles.
Destructible rocks, reeds, smoke and other aspects of the environment will make these approaches less obvious. However, opening these approaches provide some interesting opportunities. Most of them lead almost directly into the mineral line of the expansion. Fast units, such as hellions, speedlings/banelines and blink stalkers, can exploit this placement, bring some damage into the workers behind the expansion, and bug out before your opponent can retaliate. It doesn’t work for every map or every strategy, but with the right timing and units it can be devastating.
Most maps put the players in a position with a ramp that can be walled off and provides some natural defense. Other terrain features can limit the approaches of an opposing force into your bases. As many historical battles can teach us, with the right positioning and preparation, a small force can hold off a much larger one while taking minimal casualties.
Other than the immediate benefit of deterring your opponent, preparing and keeping choke points also allows one the opportunity to build in relative peace. Many players, faced with a daunting defensive position, will throw ever-growing waves of similar forces against it. While they will break through eventually, a canny player will exploit this to build a rapid response and sortie out when the next wave hits.
Of course, if either player goes for air superiority and the other is unprepared, terrain becomes something of a moot point. But that’s a consideration for another time.
* Because then, as Tu Yu observes, “the initiative will lie with us, and by making sudden and unexpected attacks we shall have the enemy at our mercy.”