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!
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!