Okay, lets put it like this.
Protoss has their photon cannons, they work like missile turrets that can hit ground units aswell. In short, battlecruisers, guardians and siege tanks can take them out before they can strike back (ghosts can too, but they suck unless you have a nuke, or is planning to lock something down for a bigger attack).
The dragoons work sometimes aswell, but they tend to be rarely used for some reason.

Terrans have missile turrets, bunkers and siege tanks as their defense. Siege tanks can take out anything on the ground, aslong as it can spot it. Bunkers usually have a short range, just like missile turrets, unless you load them with ghosts. Goliats are great in the expansion at defending against guardians though, since they have the same attack range as them with an upgrade.

Zerg does have the possibility to swarm an enemy insanely fast. Like someone said, 1 minute could we possible with a resonable game speed on a map like BGH, though it takes some luck. Most players forget to build a defense while rushing, so they can end up getting rushed by terrans if they don't find the enemy base fast enough. Their two defense "buildings", the sunken and spore colony, do a moderate amount of damage, but they both have a short attack range, like the photon cannon. Zerg's only real defense is to build up a massive amount of units that litterary crush the attackers in different ways. The scourges are great against "small but strong" attacks (BC's and Carriers), for instance.

Well, that's just my conclusion.
Happy defending.

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