Defensive Schemes

The Photon Cannon is a respectable defensive structure that has both an
anti-aircraft and ground-attack weapon. However, the Cannon is only
effective in large numbers and with unit support. Left alone, the numerous
long range weapons like Zerg Guardians, Protoss Reavers, and Terran
Siege Tanks will rip the Protoss defensive structures to pieces.

The Photon Cannon's best feature is its cloak-detecting capability. If you
want a powerful defense, the Shield Battery is the better replacement. It
will recharge your units' shields on a moment's notice. Surround a shield
battery with Dragoons, and whenever the Dragoons get damaged simply
recharge their shields-you can beat back large-scale air raids with such a
strategy. The same strategy works just as well for any other ranged unit,
such as Carriers. Unless you are massively outnumbered, Carriers
positioned near a Shield Battery (and recharged constantly whenever hurt)
should hold up against almost any attack.

Another common tactic is to make several Shield Batteries up front. Your
Zealots/Archons fighting near these batteries can be easily recharged so
long as they remain in range of the Batteries. Have a few Dragoons nearby
to fire at approaching enemies and force them to draw in nearer. Reavers
can provide great artillery support from behind, in much the same way
Siege Tanks bolster the Terran defense. Also, feel free to use Psi Storm
on your own buildings. Remember it will only damage units, so any
enemy over top or within your town is a potential target. If there are ledges
overlooking the approaches to your town (like Green Valleys) post a
couple of Templars on them. Psi Storm should thin out their ranks as they
approach.

A great tactic for late game defense is to have Arbiters over top of your
defensive units. Unless the enemy brings detectors, they'll come
streaming in without firing and be blasted to pieces by cloaked units under
your Arbiters. On the flip side be sure to have Observers or Photon
Cannons scattered about to prevent the risk of being vicitimized by
Cloaked units yourself. Hallucinate a couple copies of those Arbiters so
that the enemy can't concentrate fire on the real ones very well.

Next: Zealot Rushing