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This is a strategy to prevent your enemy from being able to move freely around the map. The advantage Zerg has is that they can expand very quickly thanks to their cheap hatcheries and fast moving units.
The first structure you want to build is a supply depot. Then build Barracks. Then build a refinery. Add a Tech Lab to the Barracks so you can start creating Reapers. When you’ve reached 50 Gas Credits, research the Nitro Packs for Reapers. You can use the Reapers to move around the map and establish some control. This is a great way to see if your Zerg opponent has started his expansion. Build your second refinery and 2 more barracks with Tech Labs. The reason for building more Barracks is that Reapers take a long time to create.
As more Reapers are created, start attacking your enemy’s base. When attacking, be sure to micromanage them. Reapers perform very well because they have a longer range than Roaches and Queens, and micromanaging is the key to taking advantage of this advantage. The last thing you want to do is waste your units sending them on a suicide mission. One positive side of using Reapers is that they are cheap to create. If played right, you will have enough minerals to build a second Command Center after a couple of Barracks.
One thing to watch out for are 4-5 Mutalisks that attack your base. A good strategy against them is to build 2-3 turrets around the Command Center.