I'm recently doing a lot of rushes, and I found that the outcome is very map-dependent.
On a map like Blistering Sands, where I know my opponent's location and he has a backdoor, how would you defend an opponent's zealot rush?
I usually set up 3 gateways in a spot near his base, that is unseen by the watch tower. The probe that builds these gateways is sent to scout, and if his main entrance is well defended I hurry up to the back door. Even if he scouts there, it's usually too late.
Best Answer
So the back door on Blistering Sands is actually covered by rocks. Since the Destructible Rocks have 2000 hit points, 3 armor and are Armored its actually very hard to get a rush in that direction.
Now if you're getting through this back door and he's not noticing the problem here is that he's not doing a good job of scouting. In the case of maps with this kind of back door you can usually accomplish this kind of scouting by placing a Pylon, Supply Depot or Overlord right next to them. If you see your opponent making a move against your back door you should be able to produce enough troops in time to mount a good defense.
In the case of a Zealot rush, you should be able to get some basic ranged to harass them while they try to break through the rocks. Alternately you could add on buildings (or even cannons) to make it really hard to get through. Controlling your Back Door is almost as important as controlling The Sweet Spot.
The Sweet Spot
Many people attack this back door early to mid game to open up what's often known as the sweet spot.
You see that area between the Back Door and the Natural Expansion? This area has a smaller route than its equivalent on the inside of the base. As a result you can quickly move back and forth before his army is able to respond.
To deal with the Sweet Spot in the early to mid game its often necessary to have your forces rally to it. By controlling the Sweet Spot you are able to prevent adversaries from abusing it against you.