StarCraft

Terran Strategy
If you happen to be playing on a Fastest Possible map, you can use bunkers to block the entry of land-bound units into your base. This, along with placing turrets on the sides of the base that can be accessed via air, is known as turtling and is effective at stopping most simple rushes.

Dark Templar Rush
Because Dark Templars cannot be seen unless in the range of a detector, they are quite useful to use before your opponent(s) get detection. However, because they are also quite high up in the tech tree (Nexus>Gateway>Cybernetics Core>Citadel of Adun>Templar Archives), it is recommended that you build some sort of defense (Cannons, Zealots, Dragoons, etc.) while teching up to Dark Templars. An early assimilator is also recommended because they cost 100 gas (and 125 minerals) each. There are too many variations of the strategy to give a specific build order.

Zergling Rush
Try to play against your opponent in a small one-on-one map that you know very well. Send your four drones to start mining minerals and order your overlord to your opponent's base (to spy on him; no race starts out with anything to attack air early; by they time a Terran player gets Marines you should have your Zerglings). Hatch 3 more drones (for a total of 7) as soon as you can and order those to mine too. When you reach 200 minerals, order a drone to morph into a Spawning Pool. Once it is done, hatch 6 Zerglings (two for each larva) and rush to your opponent. If you are lucky, you will catch your opponent totally unprepared. Try to kill as many of his workers as possible and you will be guaranteed a win. If your Zergling rush fails, you will fall behind in mining resources. Most experienced players know about this rush and can defend well against it so be careful of whom you use this strategy against.

Counters:
 * Terran - Build an early bunker near your resources and place marine(s) inside (repair it during the attack with SCV(s), or just get plenty of Marines early). Alternatively, if the map you are playing on only has one entrance to your base, you can build Supply Depots and Barracks there to "wall" the Zerglings out.
 * Protoss - Build two Gateways and warp Zealots. Most Zerg players will be able to attack you before this, so you may need to attack the Zerglings with Probes. Alternatively, build a Forge and some Photon Cannons, although this is not recommended.
 * Zerg - Do the same thing. Because your opponent is rushing you, you have extra time to build defense (your opponent must make units and have them run to your base). Alternatively, you can also get a Sunken Colony to protect your base.

Lurker Rush
Lurkers are fun to use against Terran and Protoss because of Protoss' lack of mobile detectors in early mid-games and the fact that Terran players usually use infantry in the early game. This is very similar to the Dark Templar rush (Protoss) except it is slightly lower on the tech tree but much more costly in gas. Though there are countless variations, you should probably have at least 2 Hatcheries and some Zergling support. Also, note that you should start the Lair upgrade before building a Hydralisk Den.

Counters:
 * Terran - A Marine-filled Bunker with a nearby Missile Turret placed near your resources or a choke point. If you do not have an Engineering Bay (required for the Missle Turret), you can get an Academy (which is good for the Marines & Medics and Firebat strategies) and then add a Comsat Station to your Command Center. Once you are able to see the Lurkers, Siege Tanks are good at taking them out.
 * Protoss - About two Photon Cannons placed near your resources or a choke point (if there is one) and/or Observers. Dragoons are recommended once you can see the Lurkers.
 * Zerg - An Overlord; this strategy really doesn't work against Zerg.

Mutalisk Rush
Mutalisks are fast flying zerg units and can be really annoying if used properly. They are good to use against Zerg and Protoss, and maybe against Terran too. They require a lot of gas though so build an early extractor followed by another hatchery and a spawning pool. Upgrade the hatchery to a lair. Don't forget to make more drones and overlords in the meantime. Build defenses if you have to, but not too much. After lair is done, make a spire. After the spire is done, make 6 mutalisks and rush them to the enemy base. If there are no anti-air defenses near the enemy's resources, then whoopee! Kill all his workers. You can also kill his army if his troops can't attack air.

Counters:
 * Terran - A marine-filled bunker near your resources. A large marine army with medic support is also nice. Upgrade U-238 shells and stimpacks. The stimpack upgrade is also highly useful here and can of course be used liberally thanks to the medics nearby.  A fair supply of missile turrets are probably the best defense going for Terrans though.
 * Protoss - Photon cannons near your resources. Maybe early corsairs.
 * Zerg - You own Mutalisks, and Scourge.

Higher Level Strategies
So far, you've heard about build orders and rushes and counter tactics, but what about higher level strategies, that help you decide how you should rush, or what to do with those mutalisks once you have them? Here are a few guidelines that should direct your gameplay.

Know the map
The first thing you should do, even before the game starts, is to study the mini map. If you've played the map before, excellent. Look at the distribution of resources. Are there plateaus? Does it look like there are ramps on there? If it's a ladder style map, look for all the possible starting positions, and the general locations of the natural expansions. Take a guess where your opponent(s) will start.

Be sure to take notes on what lies between you and the likely base. This should be fairly obvious, but you should know if it's possible to walk over to their base, and how long that will take. If not, early ground units should be sacrificed for air units.

Have a plan
Not having a plan is probably the reason for 99% of my losses. It is tempting to build all the way up the tech tree, having some of every unit, letting them kind of sprawl all over your base, and expanding when you run out of cash. Unplanned growth is a good way to get killed by a more skilled player. Let's take a quick example:

I played a zerg versus terran game on a small map (96x96). I knew the three possible starting points on this 3-player map, so I had a good idea of where my enemy would be and how to get there. There were quite a few choke points, so amassing a huge army was not a good idea (I'm not convinced it's ever a good idea, but it depends on the game). I alway seem to die a horrible death if the game goes on too long, so I would be attacking early. My plan looked like this:


 * Build zerglings as fast as possible.
 * Attack
 * When that fails (and it usually does) switch to lurker production
 * Build 3 or 4 lurkers
 * Attack

Now, an equally good plan might have gone something like
 * Tech straight to mutalisks, skipping hydralisks
 * Build 6 mutalisks
 * Attack
 * If that fails, tech to ultralisks
 * Build a few ultralisks
 * Ultralisk drop
 * Attack

In any case, the first plan was good enough. Even though I flubbed the build order and had my spawning pool out by the time my opponent was building his second barracks, it worked out for me. This brings up a minor point, know a good build order! You want to be able to attack/defend as fast as possible, and you don't want to spend that time trying to figure out if it's time to build a drone or spawn an overlord.

In any case, it turns out my opponent's plan was something like


 * Build a barracks
 * Scout with marines
 * Piddle about for a little bit (okay, not really)

By the time I got to him, his base was a mess. He had one barracks, an engineering bay, and a second barracks being built. My zerglings handily dispatched his construction SCV and the lone marine sitting by the finished barracks. Two more marines eventually came back from scouting, one at a time, but they too were devastated by the little insects. Eventually, he had to take many of his SCVs off of mining to fend off my guys. This setback cost him the game later on. The rest of the game went as follows:


 * He teched to firebats
 * My second wave of zerglings were mowed down
 * He started advancing
 * My lurkers arrived and wiped his infantry out as they came through the choke point
 * With resource gathering, production and unit count diminished, his base was demolished.

The moral of this story: he didn't have a plan, so he did the obvious thing: he started teching and scouting. While this is not a bad idea generally speaking, it was definitely the wrong time to do it. This early in the game, it would leave you completely defenseless and drained of resources to do that instead of getting some basic units out.

Now, it might just seem like I had a better plan, which is true. You might be asking, "How do I know if my plan is good enough?" Well, for one thing, your plan shouldn't be set in stone. Be dynamic and adapt to the situation at hand. Well, except for one element of your plan:

Your plan should always be 'Attack'
So this is really what I've been building up to this whole time. It's really the one law of Starcraft. Notice I made it ridiculously clear in my plans. Every other step was 'attack'. This isn't Civilization. You don't win a game by having a cooler culture than the other guy. You need to wipe them out, and the best way to do that? You guessed it: attacking. Do it often, and let it be the guiding force in your plan.

Here's how it works. When you attack, it's not just an exchange of blows, it's an exchange of information, and it's a two-way street. Let's go back to that example above. I attacked. Woo. That's already a path to success. By doing so, I learned the following:


 * I learned the location of his base
 * He had fewer resources, because he had obviously dumped them into an engineering bay and second barracks.
 * His units were either not being produced, or running around the map, leaving his base defended.
 * Further unit production would be slowed, because he lost some SCVs to finish off the zerglings.
 * He had not yet built an academy, or there would have been firebats hanging around.

Here's what he learned:
 * I had some zerglings and my base was to the south (or my zerglings came from the south, same difference)
 * I was being aggressive, so a second wave was likely.

So it's a form of really agressive recon. It's like scouting with the bonus of beating up a guy when you're done.

But now what? Your plan was 'attack' and you attacked. It didn't work. Well, it's back to the drawing board. What next? Well, what did you learn? He had no firebats. There's a good chance that a second wave of zerglings would work, but not for long, especially after that skirmish. So of course, his step is to defend from firebats. But for me, the beatdown would give me just enough time to get my lurkers out and about and attack again. If that didn't work, he would have turrets or a comsat station the next time I came around, so a new tactic would be required. But it did work, and I won, woohoo. It's really an iterative process.

The attacking does another thing. It puts pressure on the other player to keep up with you and keep adapting. It's an effective scare tactic. They get frazzled, and you can sit and gloat. The moment you let up the pressure, he will gather his wits, tech to something really big, and blow you away. And you don't want that.

It is almost always wrong to build a bunch of units and sit on them. That is another good way to end up hanging your head in shame. Here's what happens. You start building your units and letting them hover around in your base, or maybe on a street or back alley somewhere. Your opponent does the same, but with the pressure let up, he just builds a few guys to keep pestering you while secretly teching straight to carriers or ultralisks or what have you. It's like murphy's law in action. No matter what you build, your opponent will build bigger and better units and just wipe you off the map. You've spread yourself too thin to defend against any serious assault of one type. Sure, you can defend a bunch of minor assaults of various types, ground or air, but otherwise, you are hosed.

This is why it's important to attack. It keeps you in the loop, it keeps them off guard. You stay focused with a plan, and you build the right units for the job, rather than building everything and hoping that you have enough of each. Also, it kills dudes, and hey. That's how you win.

Cheats
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Superfast Building and Instant Upgrades
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Invincibility
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