Players have many ways of constructing structures. The fastest method is to use the hotkeys B for Build and V for Advanced Build, then pressing the associated hotkey for a particular structure while hovering the mouse near the prospected building site.
Buildings take a rather short time to build, and are rather cheap compared to units. Therefore players generally build ample buildings. Another reason of doing this is use as backup. If an enemy destroys a Protoss Gateway, for example, the player cannot build Gateway associated units nor travel up the tech tree for about another crucial minute (until a Gateway is rebuilt) UNLESS the player has at least one that was not destroyed.
Building locations are also crucial, as they may determine the ease or difficulty of bypassing units-either moving your units from one front to another or to prevent enemies from simply running past defensive structures. Buildings can be used to protect a Terran Siege Tank in Siege Mode as well as grant it extra range; enemies attacking it would be outside the 'too close' range. Meanwhile, the same buildings could block a Reaver's attack pathway or at least slow down its rate of fire.
Defensive structures should be placed at chokepoints for maximum effectiveness and should be spread out throughout the base-especially at resource nodes-to defend against enemy infiltrations and provide response time for your units.
Zerg and Creep
The Zerg is the only race that cannot build defensive structures at expansion points before beginning the construction of a Hatchery at that location. Therefore, players must defend their hopeful expansion points with an adequate supply of troops.
Creep can be a very limiting factor, but constantly trying to expand it will economically ruin a Zerg player, primarily because the creep does not DO anything. Instead of building a tangle of Creep Colonies, players should concentrate on growing Hatcheries, forming hatchery clusters at their main base. The advantage of doing so is that players have ample unit-constructing facilities for when their expansions finally begin to turn a profit.
Defensive Structure Placement
The Terrans have the least need for defensive structures, because generally a defense will include units such as a Siege Tank that cannot be defended with a Bunker. By the time an enemy has destroyed everything outside the Bunkers, the paltry resistance of Marines and Firebats will not save a base or expansion.
Zerg players turn almost all creep colonies into their derivatives, because doing so only costs an additional 75 Minerals, compared to the 150 for a Photon Cannon.
Main Structure Placement
The Tech Tree related structures of each race, if seen by an opponent, will reveal to the opponent all of a player's buildable units, which reduces the element of surprise and allows the enemy to concentrate on units that are effective against the player's own units rather than spreading out their military variety to cover every contingency.
For example, a Terran Machine Shop will alert any enemy to the production of Terran Siege Tanks and will result in their constructing units such as Zerg Queens, Protoss Archons, and any type of aerial units, all of which are effective against the Siege Tank for their own reasons. Then if the player were to invest in Siege Tanks, the enemy will be prepared. However, if the player instead chooses to build anti-air units or the Terran Science Vessel with Irradiate, which are effective against the Queens, Archons, and air units, then the advantage will return to that player.
Main Structure Backup
Oftentimes opponents will attempt to destroy lone key structures, such as Gateways, Cybernetics Cores, Factories, Starports, Spawning Pools, Spires, and Ultralisk Caverns in an attempt to cripple a player's ability to produce a certain type of unit or advance to the next step of the Tech Tree. Since a player's base is often well-defended, destroying such a structure rather than attacking units will bring a significant cost on an enemy's resources. Therefore, players may trick them into an attack with a lone obvious target, then produce a second one hidden deep inside a secure base. Then if the enemy attacks, the player will still have the backup, and the enemy attack will be wasted. Note that Zerg players are most significantly affected by the loss of a Tech unit, since they often have only one of a Spawning Pool, Hydralisk Den, Spire, Queens Nest, Ultralisk Cavern, and Defiler Mound.
Terran Structures as Flying Cover
By Flying Cover we mean the use of a large floating structure to defend a smaller ground unit(usually a Barracks or Engineering bay is used because they are mainly used to teching). Because of the gameplay, player opponents cannot target any unit completely hidden beneath another unit. With the Terrans' flying structures, this can become a significant benefit in the defense of a base because unless an enemy uses splash damage attacks or spells such as Psionic Storm, they cannot target the covered unit until the Flying Cover is destroyed. Destroying a Flying Cover Barracks with 1000 hp is much much harder than destroying a Battlecruiser with 500 hp.
The obvious counter to Flying Cover is to Stop all units and have them automatically attack the Covered unit. However, doing so is not effective because the units may not attack that particular target, and almost never will if the Covered unit is a spellcaster such as a Terran Ghost.
Flying Cover with floating Terran structures is most effective when the Barracks or Factory is directly adjacent to the defending military and the military is slightly above the structure. That way, as the battle opens up, the structure floats up and immediately blocks the view of the Covered units.
The same strategy can be used to covers a player's buildings thus disallowing the opponent to see how they are teching or what they are planning. For example, in one game, a player floated his Engineering Bay over where his Comsat Station add-on would go, and instead he built a Nuclear Silo in complete secrecy. He later launched the nuke successfully at an unsuspecting player.
The Protoss really only have one type of defensive building: the Photon Cannon. It's expensive when compared to a missile turret or a derivative of the Creep Colony, but it accomplishes both anti-air and anti-infantry in one unit. In a level such as Lost Temple, where the only way to get to an opponent's base by land is through a single ramp, blocking off the ramp with as many Photon Cannons as possible can be a very effective method or defense. Building one every couple of grid units around your base can also counter any air units or cloaked units that may assault your base.