Game Guide: Choke Points
A choke point is an area of the map that is surrounded by impassable barriers. To go through a choke point, a group of units will funnel into a narrow opening, which will limit their mobility and make it harder for them to attack.
Using choke points to your advantage is a very important tactic in StarCraft II. Defended choke points can prevent enemy units from reaching your base, protect vulnerable workers, or even allow you to take on a large force with a small group of units. Ultimately, however, the location of the choke point dictates how effective this tactic will be. The top of a ramp is an ideal location to build, because you also have the advantage of higher ground. "Walling-in" revolves around artificially narrowing natural choke points (like ramps, for example) by using certain structures. Learn more about walling-in.
Each race in StarCraft II employs choke points in different ways. Terran players can use bunkers, supply depots or a natural ramp to create highly defensible positions. Supply depots can also be lowered or raised to serve as gates. Let your troops in, keep the enemy out.
Protoss players employ regular buildings, photon cannons, and the sentry's force field ability to block off access to their bases.
Zerg players typically use their units and spine crawlers to protect their natural choke points. They can also use the burrow ability to set up ambushes, divide the enemy army (burrow in the ramp, let several marines pass through, and unburrow to prey on the tanks behind), and catch opponents off-guard.