Recently evizaer wrote a post over at That's A Terrible Idea titled "Air Units in land-based RTSes". In it he wrote about how air units in RTS games are "awkward". Saying they're either to powerful and become the only option, or they're just "ground units that ignore terrain". He goes on to wrap up his post by saying, "making all air units off-map call-ins tremendously increases the seeming realism and fun of air units while doing nothing to damage the metagame."
This finishing statement, in my opinion, is completely and utterly wrong, and what better way to prove my point, than with Starcraft 2.
Starcraft 2's air units fall into evizaer's category of "ground units that ignore terrain". On a surface level, this is true. However, there is much more to air units in Starcraft 2 than what that statement lets on.
There are two states in Starcraft 2, when refering to air units, that a unit can be in. These states are, of course, air and ground. All units have to be either one or the other, or, rarely, both. Ground units act as all well behaved ground units should and obey the laws of the terrain. Air units, as already mentioned, ignore the terrain.
While air units aren't that much faster than ground unit, this ignoring of the terrain make it so they can easily attack the back of a base when the enemy isn't expecting it. When defending against air units, it forces the player to rethink their defenses so that they encompass their whole base and not just a choke point.
But these states hold more significance than just terrain rules. They also determine what can attack what. In Starcraft 2, weapons can attack either ground, air, or both. This can create a weird kind of, for a lack of a better term, rock paper scissors effect. It allows for situations where air units wreak havoc when an opponent only has units that attack ground, or ground units that steamroll because an enemies army can only attack air. It makes powerful defense units, like the Siege Tank, have a weakness because they can't attack air. It adds a whole new layer of thinking when selecting which units to produce.
Lastly, I want to cover a concern evizaer brought up.
"When given the viable option at the beginning of a match, a player should almost always choose air units before they begin to use ground units to cement map control. An air unit that is equally as effective as a ground unit at ground attack is significantly more valuable in that it can ignore terrain to harass the opponent from any angle. Since games have a sharp divide between units that can shoot air units and units that cannot, the early game units generally are putrid at air defense. If they were good at air defense, then air would never be a viable strategy because building basic units would hard-counter it."
About air units being the only viable option in Starcraft 2, this is false. There are two things that stop people from massing air unit and getting away with it. One, air units are expensive, and when I say expensive, I mean very expensive. Two, if your opponent catches on that your massing air units, it is very easy to produce a counter force that will annihilate all the air you've been producing.
As for air units being equally effective as ground, this is also false. Air units in Starcraft 2 fit into niche roles. Banshees do massive damage to single ground units and are great at harrass, but can't attack other air units. Void Rays destroy heavy, high health units, but suck dish water against a group of low health units. Mutalisks tear through groups of light, small units, but fall quickly and easily to AoE damage. No unit is 100% foolproof in Starcraft 2, air units included.
Finally, about basic units not working against air, Starcraft 2 does something amazing that not many RTS games accomplish. Depending on what your opponent builds, every unit is worth getting throughout the whole game. As for air units, the example that stands out in my mind is the Void Ray. Void Rays will fall very quickly to a handful of marines, which are the first attack unit Terrans can build.
To wrap it up, everything that I've mentioned here adds layers and layers of strategy that changing air units into off-map call-in abilities would not. As for the off-map call-in abilities, that air strike could be an artillery barrage, the air drop could just be teleporting supplies in, and that recon plane could just be a satellite scan. And when you put it that way, you're back to having no air units in your game at all.
This post originally appeared on Lost in Neurons Wednesday, August 4, 2010. As part of a clean out of Lost in Neurons to make it more focused, it has been moved here, where its author feels it better fit.