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Defending an Island that a Protoss is Already Defending is Surely Suicidal

The Infamous, and Often Dreaded "Cannon Push"
by Zileas

The cannon push is essentially a method of quickly securing an enemy expansion as your own.  Essentially you land some troops, usually a combination of templars and other units (depending on what the enemy can field against you (usually a reaver or two a zealot or two some templars and an archon works nicely) and quickly establish a few shield batteries and a good half-dozen cannons. This strategy is especially effective against mid sized and large islands, but can be used on land especially around crucial choke points.  You can also implement this AS you are doom dropping a very small island, but then again, thats essentially just a doom drop.

The idea behind this is that protoss troops with shield batteries and cannons are nearly impossible to displace, especially by zerg and to some extent protoss.  Do not try this against a terran on a large island.  It works fine on a small island if youve killed their tanks however, but usually a doom drop is required since most terrans will madly turret up.  If you catch them WHILE they are building the exp, this strategy works fine though.  Anyhow, back on track.

Against Zerg and Protoss mix in a few scouts to deal with airial siege units (guardians, carriers), and they are basically screwed.

As the name implies, you push with the cannon network.  First you set up your cannons, and of course defend it.  After this, you just push right into their base by setting up pylons forward, and setting up new sets of cannons, while covering the ground with your reavers and templars, and dealing with air units with cannons and templars.  Shield batteries should keep your troops alive pretty easily.  This tactic, once started, requires HUGE resources on the part of your enemy to stop, so generally you end up with a ress dry zerg or protoss, or one without a main base or expansion.  Either way its cost effective.  The other reason it works well is that minerals are cheap to a Protoss -- gas is always the limiting factor.  Squandering 3k minerals on cannons and batteries and plyons really isnt a big deal if you are kicking them off a key island, or razing a main base.

I guess another way of cosndering this is that Protoss cannon/templar fields are REALLY hard to destroy and very easily defended.  If you set up such a thing nearby an enemy base, and expand it into it, you effective have made THEM The intruder on your island.