Before we discuss counter builds, let's talk about what an early Zealot rush is:
"my defenders got wiped by 4-5 zealots and more and more zealots kept pouring in and eventually killed me"
This is the dreaded 2 Gate rush.
Your first goal is going to be identifying it. Luckily this is fairly easy to do. If you scout with your 12-13th drone/probe/scv you should get to your opponents base while his first Gateway is still building, worst case while his second is. All 2 Gate rushes start the same: Pylon, Gateway, Gateway, Pylon. This makes them very easy to pin down. If you see that second Gateway go up before the Cybernetics Core you know he's playing some sort of rush (pro tip: you can click on a Protoss building warping in to see what it will be). If he does a delayed rush, chances are you'll already be in a better position to defend, so you really want to watch out for this early rush.
The attack that will come should be 5 Zealots with 2 more in route (obviously he may wait for more or less, but this is the best time for him to attack).
As Zerg a 2 Gate opening is one of the scariest things to deal with (right after Hellion harass into Mech). If you fast expanded, use the expo as a damage soak and cancel it before it gets killed (if you can). Ideally you'll want to keep that expansion alive as his economy will already be devastated. When you scout a 2 Gate, you'll want to build a Spine Crawler almost immediately. Additionally, a Roach Warren is also recommended. Roaches are very strong vs Zealots (slow attack and starting armor vs a unit with fast attack) and if you can position some by your ramp or other choke you can cut the surface area of your Roaches in half. If you're already into a more speedling based build, then you're going to want to wait for him to surround your Hatchery or engage your Spine Crawler. This will naturally give you a very nice surround on his units as they will spread out. I also recommend bringing your Queen along for ranged dps (and to infuse the Spine Crawler).
The key thing for Zerg about repelling a 2 Gate play is the recovery. Early 2 Gate is so effective that even though he is putting himself at an economic disadvantage, he may do enough damage to your economy to win. You'll want to do the following: Put an Zergling outside his based (this is a scout for his next move); Tech to Roach or Hydra or Muta, as 2 Gate will likely lead into 3 Gate, 3 Gate + Robo or 4 Gate; Power drones, this one is key as he's making a huge economic sacrifice to push with 2 Gate and you need to exploit this for victory.
Terran traditionally have less problems with 2 Gate as they can usually wall off before the Zealots get there. If you already have Vespean Gas then its ok to go for Marauders with Concussive Shell, however, if your strategy didn't include that, you're going to want to skip add-ons and throw down a second Barracks when you scout the second Gateway. With Marines faster build time, you can create a tough Marine force behind your wall well before his Zealots arrive. This ruins the fact that he made an early sacrifice to go for a Zealot push and should give you command of the match. From here is very easy to transition into an MMM or even BioMech which should give you a quick victory.
As Protoss you're going to also want to wall, stick a Zealot or two in the opening and push for Stalkers. If you can have enough 1 Stalker out and your opening plugged before he gets there, chances are you've already won. It is not unusual, upon seeing a 2 Gate, to throw down a second gateway yourself. As defender you'll naturally have the advantage as his troops have a longer rally distance, and you can control the choke. If you push out an early Sentry, he'll probably Rage Quit and call you a cheater.
Now if you're dealing with a Proxy Gate (which is different than a 2 Gate) you have slightly different things to watch for. Lack of Assimilators and Pylons in his base are your best clue. At this point you'll want to start taking your scout and covering all the corners of the map. Additionally move a scout to outside your natural or the nearest Xel Naga tower.
It sounds like you're building a large army and using it to gain map control, but then are falling prey to things like drops and backdooring.
If you have map control your first thought should never be: Crush my opponent, as you're essentially sacrificing an advantage (map control) for an unsure victory (because he doesn't control the map doesn't mean he can't take you). Instead your thought should be: take the map. I think you'll find this change in philosophy will lead to situations where you have a much more powerful economy than him and you're able to kill him with attrition.
Now if you're falling prey to things like BD here is a quick check list for you:
- Are you scouting? an Observer should be the first thing you build out of Robo. If you don't go Robo, a Hallucination works just as well. Sometimes scouting means sacrificing some Zealots in a pointless charge to just get inside his base.
- If you're scouting, are you seeing it coming? Overlords on the edge of your base, Starport with Reactor + 3 Rax, all of these are indicative of certain strategies you should watch out for.
- If you see it coming, how are you responding? If you have map control that means you should be able to respond quickly. Shutting down these back door attempts is often a bigger victory than a straight up conflict
- Finally, Do you have map control? If you don't have map control that means he does, and if he's beating you with map control then your problem is not "how do I deal with a base trade?" but rather, how do I regain the advantage.
Best Answer
It mostly comes down to larva and the inefficiency of Zerg units in larger numbers as well as the gas requirement for more efficient lategame units.
Zerg need larva to produce, larva comes from hatcheries, 3 hatcheries with good injects are about equal to 2 base terran or protoss production and if you are building hatcheries anyways, you might as well place it at a new base.
With sufficient creep spread (which is extremely vital) and good scouting the opponent won't be able to kill it off early game anyways.
If there is the chance that he might kill it, for example when you scout some sort of all-in, it's ok to get the third hatchery in your main or natural (aka macro hatch) and delay the third base but usually you are safe enough to just take that base.
In midgame the cost efficiency of Zerglings and Roaches falls off rapidly, which means you either need a lot more of them or you need a lot of gas for higher tech units (point 3 below). A lot of units means a lot of larva and resources which means you need an extra base.
For your mid- to lategame units (mutalisks, infestors and broodlords) as well as the very critical upgrades you need tons of gas, which means you have to get your 6 extractors relatively early to build up the gas so you can get out a critical number of units once your tech structure/research finishes.
As for having to pressure with mutalisks you might want to watch Stephano who has a playstyle that completely skips those and goes from infestors straight to broodlords. Considering that on the TeamLiquid TLPD he's currently rank 1 worldwide it doesn't seem to be a bad style either. Many people who play the mutalisk style get stuck there since they use all the gas for mutalisks and then don't have any gas for higher tech units like broodlords that you really, really need in lategame, especially against protoss.