Each race in SC2 has its own mechanical quirks quite separate from individual unit balance. Terrans have their addon swapping and mules, Zergs have larvae injects and creep, Protoss have warpgates and chrono. The most profound effect of the warpgate mechanic is not defensive in nature, but offensive. To find out why, we must examine the concept of the defenders advantage.
A primer of defenders advantage
When playing as a Terran or Zerg, you naturally have several advantages as a defender before you even build a unit. First, your ramp - the narrow choke allows your units to have a concave against a bunched up pack trying to push its way up the ramp. Second, your production - your rally distances are much, much shorter than if you were out in the middle of the map, so you can produce less units and still defend, because your second wave comes quickly.
Also, both Terrans and Zergs have access to a defensive structure after building their first basic unit production structure. The spine crawler only requires a spawning pool, and the bunker only requires a barracks. These structures can be produced as a reaction to a scouted push in order to provide you with an additional advantage as a defender, which can help mitigating the loss of the ramp advantage when you expand.
Defending against a push
Consider what happens when defending - assuming perfect balance, the only way to survive a push with an inferior army is to abuse your defenders advantages: your quick reinforcements, your ramp, and any defensive structures. This is why as a fast expanding Terran (no ramp advantage) you construct bunkers against a Protoss gateway timing. Same goes for spines as a Zerg.
But what about Protoss?
Leveling offense and defense
The warp mechanic, by its very nature, crosses "short rally" off of the defender's advantage list for Protoss. An attacking Protoss army and a defending Protoss army will be reinforced just as quickly.
And here comes problem number one: in a PvP, the defender has essentially NO advantage besides the ramp (and on Tal Darim Altar, there is no ramp!). The only way to survive a gateway push is to match your opponents unit count or abuse the crap out of your ramp (sentries, hooo).
This is how the 4gate v 4gate metagame evolved, especially on TD Altar. Cannons represent an absurd deviation from normal tech in order to provide a defender advantage, since there is no defensive structure after gateway. Scouting a forge + 2 cannons (450 minerals) can be responded to with a free expansion (400 minerals), since the cannoning player theoretically cannot attack with more units than the expanding one.
But PvP, as a mirror matchup, is inherently balanced no matter the design flaws, so I'll ignore it for now. Still, there are other implications of the Protoss equivalency of offense and defense.
The three types of engagements
There are three fundamental types of engagements in SC2. It's basically common sense, but it'll help to give them names. When you do a (1) timing push, you have usually sacrificed a small amount of economy for a stronger army at a specific time in the game. Therefore, when you push and engage, you have a stronger army then your opponent. This is the first type of engagement. If you are (2) defending a timing push, your goal is usually to use your smaller army together with a defenders advantage to defend and later capitalize on your stronger economy. This is the second type of engagement. Finally, in the lategame, midmap engagements are commonplace, where both players have relatively evenly matched armies. We'll call this an (3) even fight.
As mentioned before, Protoss has a natural advantage in timing pushes because of the warp mechanic. I feel that if a Protoss is engaging in a timing push, game feels well balanced - extremely strong Protoss timing pushes can punish greedy Terran or Zerg play, and well executed defenses can hold in safe play.
A pushing Protoss, with a stronger army and a short rally, is meant to have a fair shake at attacking a smaller, but defensive structure and rally fortified Terran or Zerg.
As a defender, Protosses have cannons for defense, a rally advantage, and a ramp / choke advantage. Cannons, because of their sometimes inconvenient tech, are missing from a Protoss defense in a lot of earlygame scenarios (4gate, 2 or 3rax, roach+ling aggression on expo). This reduces Protoss defensive options to a ramp or choke advantage, and a rally advantage. Protosses can defend using the choke advantage very well, by using forcefields. However, on maps with open expansions, this becomes extremely difficult. This is why Protosses prefer maps with narrowly choked off naturals, like Shakuras, or Anitga. On open maps, the choke advantage disappears almost completely.
So what about the rally advantage? Here's the problem. Because of the warp mechanic, Protoss is balanced as if they have the rally advantage for a timing push engagement. In a defensive sense, the warpin mechanic provides no additional benefit compared to an offensive one. As the Protoss is playing defensively, they have a smaller army compared to the pushing player. The choke advantage is missing at the natural on many maps. Cannons are absent because of tech inconvenience. All that's left is the rally - which is designed to be fair for a PUSHING Protoss!
A defending Protoss, with a weaker army at home, has no significant defender's advantage over a timing push from the opponent, making the battle favor the pushing player.
The differences in the races amounts to basic units and their defenders advantage. A Zealot, Stalker, Sentry army is equally good on offense and on defense. A Marine, Marauder force is good on offense, but BETTER on defense, because of bunkers and a shorter rally compared to their offense. Same goes for Zerg - a force at home is BETTER than an attacking force because of creep, spines, and relatively short rally. This lets you open economically and defend with a smaller, but advantaged army. A defending Protoss army has no advantage compared to an attacking one, no edge to capitalize on. So, if a Protoss early expands, their weaker army cannot make up for their lack of size with any external forces like a Terran or Zerg one can, and is vulnerable to timing pushes from the enemy.
It just comes down that unit efficiency. In a defensive context, a Protoss gateway army is weaker than a Terran or Zerg army. I'm not saying that gateway armies are weaker in general! Just in a defensive context, in terms of efficiency. As an example, if you were trying to hold your natural expansion on Xel Naga Caverns against light pressure, which would you prefer - 1 sentry, or a bunker with 2 marines in it? 2 sentries and a stalker, or 2 full bunkers? The options cost the same (1 gas = 1.5 min), require the same tech (actually toss requires more tech - gas and cybercore). I think its clear that the Terran options are extremely superior.
As a result of this, Protoss expansion builds, in order to be safe to strong early timing attacks, must have one of the following characteristics to provide the necessary missing defenders advantage:
- An early forge (FFE v Z, delays tech because of forge tech being out of the way... imagine expanding as Terran by going ebay+turrets... tech and units for your own offense are going to be slow!)
- A map with a choke for sentries to use (Shak, Antiga, this is by far the best option)
- A lot of units (3gate exp, sacs econ compared to other races expo builds)
- TL;DR:
- An offensive and a defensive Protoss have the same rally distance.
- Protoss timing attacks with large armies are balanced against defensive, economic Terrans and Zergs (both races have economic openings that are ahead after defending a Protoss timing, but there is potential to do damage against a greedy opponent).
- Because of these two points, a defensive Protoss with an small unit count and economic opener is weak because what is normally a defenders advantage is not a defenders advantage for them, it's a given in both offense and defense.
- The lack of a defensive structure after gateway adds to this problem.
- Therefore, the lack of a strong defenders advantage means Protoss has no safe, economic openers.
As an aside (not to be taken too seriously),
To fix this:
Add additional defensive building. A buffed shield battery could be awesome as a defensive tool if it were available after gateway. Not necessarily a clone of the BW one, maybe something more like a stationary medivac for shields only.
ADDENDUM:
+ Show Spoiler +
Ok here, look at it this way - Expand vs Pressure builds.
A) Look at a Protoss timing push vs a Terran expand build. Protoss 3gate pressure (into expand), vs a Terran 1rax gasless expand into 3 rax.
Terran expands, scouts the 3gate pressure, and bunkers up. Would you say this is a fair fight? Protoss is probably going to get repelled, but theres a chance that they can break it if the Terran is sloppy. It can go both ways, theres tension in the matchup, and it feels balanced. If the attack fails, Protoss is behind, and Terran has defended well. If the attack does damage, the attack has succeeded, and the Terran is behind.
What's actually happening in this example is that the Terran is compensating for their smaller army (since they expanded first) by using a defenders advantage - the bunker with repair. The salient features are:
Protoss has a larger army (expanded later, pumped units early)
Protoss has a short rally (warpin)
Terran has a small army (expanded first, units later)
Terran has bunkers (defenders advantage)
Terran has a short rally (home base)
And this SET of features creates a fair fight.
B) Now flip the roles. Terran's doing a 2rax pressure (12 + 16 rax, 1 tech 1 reactor, concussive researched) vs a Protoss 1gate expand into 4gates.
These are more or less equivalent builds to the previous example, except its 2rax+addons which is slightly cheaper than 3gates + cyber. Anyways, Protoss scouts the 2rax. The Protoss, on 1 base with a nexus building and 4gates on their way, cannot get cannons up in time. No defensive structure is available, and the natural is wide open. The Protoss expanded off 1 gate, so they have at most, 3 units when the push hits (Stalker Sentry x2 usually), with no repaired bunker to fall back on.
I think we're all familiar with this situation. MC lost in this exact situation to Polt. This fight is NOT fair, its almost a build order loss. You either sac your nexus and abuse your defenders advantage (ramp + sentry), or SEVERELY outmicro your opponent. (Or you could be on Shakuras and you can FF your natural. Which is why I stated in the OP that these maps are good)
Again, examining the salient features:
Protoss has a smaller army (expanded first)
Protoss has a short rally (home base)
Protoss has no defensive structure after gate
Terran has a larger army (units first)
Terran has a LONG rally (attacking)
And this SET of conditions results in a Terran gaining an advantage the majority of the time.
My argument is that an economically focused, defending Protoss, when they engage, has essentially the same characteristics as a Protoss doing a timing attack with a small army... which is, of course, a terrible idea, and results in losses.
A) Look at a Protoss timing push vs a Terran expand build. Protoss 3gate pressure (into expand), vs a Terran 1rax gasless expand into 3 rax.
Terran expands, scouts the 3gate pressure, and bunkers up. Would you say this is a fair fight? Protoss is probably going to get repelled, but theres a chance that they can break it if the Terran is sloppy. It can go both ways, theres tension in the matchup, and it feels balanced. If the attack fails, Protoss is behind, and Terran has defended well. If the attack does damage, the attack has succeeded, and the Terran is behind.
What's actually happening in this example is that the Terran is compensating for their smaller army (since they expanded first) by using a defenders advantage - the bunker with repair. The salient features are:
Protoss has a larger army (expanded later, pumped units early)
Protoss has a short rally (warpin)
Terran has a small army (expanded first, units later)
Terran has bunkers (defenders advantage)
Terran has a short rally (home base)
And this SET of features creates a fair fight.
B) Now flip the roles. Terran's doing a 2rax pressure (12 + 16 rax, 1 tech 1 reactor, concussive researched) vs a Protoss 1gate expand into 4gates.
These are more or less equivalent builds to the previous example, except its 2rax+addons which is slightly cheaper than 3gates + cyber. Anyways, Protoss scouts the 2rax. The Protoss, on 1 base with a nexus building and 4gates on their way, cannot get cannons up in time. No defensive structure is available, and the natural is wide open. The Protoss expanded off 1 gate, so they have at most, 3 units when the push hits (Stalker Sentry x2 usually), with no repaired bunker to fall back on.
I think we're all familiar with this situation. MC lost in this exact situation to Polt. This fight is NOT fair, its almost a build order loss. You either sac your nexus and abuse your defenders advantage (ramp + sentry), or SEVERELY outmicro your opponent. (Or you could be on Shakuras and you can FF your natural. Which is why I stated in the OP that these maps are good)
Again, examining the salient features:
Protoss has a smaller army (expanded first)
Protoss has a short rally (home base)
Protoss has no defensive structure after gate
Terran has a larger army (units first)
Terran has a LONG rally (attacking)
And this SET of conditions results in a Terran gaining an advantage the majority of the time.
My argument is that an economically focused, defending Protoss, when they engage, has essentially the same characteristics as a Protoss doing a timing attack with a small army... which is, of course, a terrible idea, and results in losses.