Photon Cannons have a relatively slow rate of fire and are thus very susceptible to swarming attacks. Large groups of small, expendable units like Zerg Zerglings or Terran Marines can easily overwhelm Photon Cannons with minimal casualties on their part. Further, certain types of units including Zerg Guardians, Terran Arclite Siege Tanks in siege mode, and Protoss Reavers attack with superior range to the Photon Cannon and may eliminate them safely. Terran Battlecruisers can crush Photon Cannons with a single blast from their fearsome Yamato Guns. You must understand, therefore, that Photon Cannons alone are not sufficient to defend your base. You must support them with mobile ground forces backed by Shield Batteries, in which case Photon Cannons will provide useful cover fire. But on their own, your Cannons will not last long.

Photon Cannons are at the mercy of the Pylons that provide them with power. Don't cluster several Photon Cannons about a single Pylon; the opponent will simply eliminate the Pylon, rendering all your Cannons inoperative. Use close pairs of Pylons to provide reinforced power to your Photon Cannons.

Photon Cannons are useful for their cloak detecting ability. Units like Terran Wraiths and Ghosts cannot sneak past these defenses, so place Cannons in areas where you anticipate these kinds of attackers. Ultimately, Photon Cannons are a useful deterrent and are especially effective on raised plateaus inaccessible to ground forces. Place them throughout your base when resources allow, but know that you must be ready to back them up should an opponent storm in with a sizeable force.

Next: Advanced Protoss Structures