Because the battlefield is not a chessboard, one must always exploit
the terrain to their advantage to win in StarCraft II.  Two
such pieces of terrain commonly used are the Choke Point, and a player
created Wall-in.    These will be run into from most
players on an every game basis, and dealing with the effectively is
required or you'll have to exert far more force in breaking them, or
even give them an opening to turn the tides on you!

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style="font-style: italic;">The big thing to consider about these--do they actually have a lot of force behind them? This is ripe for the picking!

Choke Points - obligatory 'the neck lol'

The loose definition of a choke point is any point where units are
funneled into a more narrow area.  These exist often along the
exit path to your natural expansion, and are a key area to hold.

 

The main purpose to holding these is the ability to set defenses and
units in the wider area surrounding it.  Your main goal when
fighting around these is to take your opponent's cluster of ranged
units and have them clog up the choke point.  That will force
your opponent to do one of two things to get all of his units in firing
range--move forward, or move back.  Moving forward means free
shots for your whole army.  Moving back means you probably
took out a few units with no losses yourself.  Some units and
abilities are exceptionally powerful  in these situations.

 

Terran
-  Siege Tanks in Siege Mode, Ghost EMP, Hunter-Seeker
Drones all rip up clusters of units with their splash damage.

 

Protoss -
Psi Storm was designed to ruin these fights. 
Colossi with the range upgrade also make suitable units for these jobs
as long as something is in front of them to hold them in the choke
point.  Sentry Force Fields are exceptionally powerful as
always here.  Hell, you can use a Mothership Vortex to remove
the vast majority of their army more than likely.

 

Zerg -
Banelings and Infestors make for a powerful choke point offense
and defense.  Banelings simply require the opponent to pay
attention and micro very carefully... because the stakes are much
higher than before due to the units being shoulder to
shoulder.  If they get the idea of retreating or chasing a
target with micromanagement, Fungal Growth will see to it that this
choke point becomes their grave.  Brood Lords can be good, but
make sure they're targeting units in the middle or back of the
opponent's army to ensure that the Broodlings aren't getting in the way
of your other melee units if you're using Zerglings.

 

Wall-ins - no rush 20 please

Almost always seen from a Terran, sometimes from a Protoss, and it
takes a special player to try and do a wall-in with Zerg, this tactic
is all about denying access.  Traditionally it involves a
Barracks and Supply Depots, and in StarCraft, when you were ready to
push out, you just lifted the Barracks to create your exit. 
If you are going to do one of these, do it very quickly.  If
you can successfully deny your opponent's first scout from entering
your base, you will have the upper hand.  You can have hidden
tech buildings, suddenly throw up 3 additional Gateways when they give
up and leave, or even expand.  Denying your opponent
information is a major benefit of the wall-in.

 

But, you do have to punch through it a lot when facing Terrans, so lets
look at what we can do to punish enemies that would hide from us.

 

Terran - Very carefully placed Siege Tanks can pelt the walling
structures with shells, while being out of range of defending Siege
weapons.  The Yamato Gun is exceptional at breaking walls and
defenses powering them, so it might be worth the research.

 

Zerg - Sending in a quick group of 5-6 zerglings to distract while
sending 6-7 banelings at the walling Supply Depots or Pylons will
result in a big gaping hole in your opponent's base.  Exploit
it!  If you can't pull this off, your best bet is going for
Mutalisk harassment into Brood Lords to siege it out.

 

Above all else, be patient when attacking and defending both of
these.  If you get caught in a rough spot, you can lose any
advantage you had instantly and even be at risk of a counter
attack.  If your opponent is hiding behind either of these all
game, it's a license to expand for you.  Eventually, they'll
have to crawl out from their wall-in or choke point to seek additional
resources.  Hopefully, all they find is you with a vastly
superior army since you control the map.  If you can't go
through them, remember you can go around them with paratroopers from
the skies!