Game Guide: Protoss Photon Cannons

Photon cannons are useful in both offense and defense. They also serve as stationary detectors that allow you to see enemy cloaked or burrowed units. In order to build cannons you must have a forge.

Defensive Cannons
Photon cannons are a great addition to your defenses. Make sure they are powered by multiple pylons whenever possible, this way your cannons will remain operational even if the enemy targets your power grid. Place a few photon cannons around your minerals and gas to stop attacks from cloaked or air units, and alongside the edges of your base to discourage drops.

This base has a good starter defense against random attacks. Additional pylons are recommended.

This player has a nice base defense.

Cannons are often essential to stopping early game aggression when fighting zerg. They are also a key component of the forge fast expand, which is detailed in the Ramp Defense section. As the game progresses, adding cannons to your expansions can prevent zergling run-by's from crippling your economy when your main army is unable to deal with them.

Offensive Cannons
“Offensive cannoning” is part of every Blizzard RTS, and StarCraft II is not the exception. You will need a pylon and a forge to build photon cannons. In later stages, it is possible to use warp prisms instead of pylons to generate power fields to construct your cannons.

The purple protoss hid a probe in the smoke, then slowly built some photon cannons out of range. Once four photon cannons were ready, it was time to build further into the enemy blue protoss base.	A player builds their forge and cannons right outside the enemy base. The attack failed but the terran didn't destroy the forge. Though left unpowered, it still allowed the protoss player to construct cannons back home.
A protoss player sets up cannons right in front of the enemy's ramp. It proved difficult for the zerg to exit the base.	A probe tries to “offensive cannon” a zerg base. Unfortunately it got spotted by an Overlord.

Offensive Cannon Rush
The cannon rush is a staple early game “all-in” build that every player should know how to recognize. First, the protoss player sends a probe to the opponent's base right after placing the first pylon. Then builds a forge in his base and places a pylon close to the enemy base. From that point on, the protoss player works his way into the enemy base by adding more cannons, always in range of those previously built, until they can fire upon the opponent's structures. A few well placed static defenses will stall your opponent, buying you precious time to regroup and devise a counterattack, but you might not be able to do anything when your base is surrounded by a sizable amount of cannons. The key to defeating this strategy is, and always will be, proper scouting. Never allow your enemy to build anything close to your base, much less box you in.  

Protoss builds outside a zerg base.

Protoss succeeds in defeating the zerg.

More Offensive Cannoning
Player bases are often vulnerable to assaults from high ground positions. You can load a single probe in a warp prism and fly over to the enemy base. Use the phase prism's phasing mode and build a cannon. From that point on you can expand your cannoning, or even warp-in additional units.

The start of the attack using just a probe and a warp prism.	More pylons and photon cannons underway.
Stalkers can now be warped-in.