Microsoft's success at breaking into the top tier of the RTS world may have been unsurprising, but Cavedog Software's wild success with Total Annihilation (1997) was much less predictable. In this case, the game's success has as much to do with its excellent gameplay as with marketing and corporate clout. Yet Total Annihilation is fairly similar to Age of Empires in one key respect: it's epic. Both games bring many more units to bear than previous RTS games, and both have a scope well beyond even the alternate history and fantasy world wars of C&C and Warcraft. Where Age of Empires was an epic covering the scope of early human history, Total Annihilation was a science fiction epic spanning the galaxy. It's clearly inspired by Star Wars, as just a few notes of the bombastic soundtrack make clear.
The premise of Total Annihilation is thin for supporting a story, but wonderful for supporting a setting: a group of supercomputer AI's called “The Core” have mostly taken over the galaxy, whereas some human rebels known as “The Arm” decide to fight them using armies of clones. In practice, this means massive armies of mechanized/robotic units, battling over a wide variety of planets, from beaches and lava to lush forests and cities which span entire planets. Nothing in the premise suggests anything small-scale or light-hearted, and nothing in the game threatens the perception of epic scale.
The key to making that work as a game is having enough units available, both in choices for building and varieties to create. Total Annihilation's claim to fame is its selection of hundreds of different units. The Arm and Core units are generally mirrors of one another in effect, but have significant visual difference. Each has dozens of different units, from light infantry to huge capital ships to speedy aircraft. Some are cannon fodder, others are elite units, built to last. The mass of different kinds of units replaces unit upgrades as well.
Even more impressive than the amount of units available is the fact that you can built them, in the hundreds. Many other RTS games limit population growth as best they can via artificial caps, such as the farms and houses that need to be build in Warcraft and Age of Empires. Total Annihilation, on the other hand, allows you to build as many units as you can, so long as you have the resources to cover it. Interestingly, its resource system is not based as much on what you have at the moment, but rather, what your overall flow of resources in and out is. You can build as many units and buildings as you want at a time, but if you aren't also getting enough energy and metal, your building will stall as they are collected. Most RTS games refuse to let you even start building troops or buildings unless you already have the necessary resources.
Moreover, it's easy and fast to build multiple things at once. Shift-clicking, which sets waypoints for movement in many games, can also be used for collection and building purposes in Total Annihilation. You can tell one of your builders to recycle metal from a handful of destroyed units, build several laser turrets for protection, harvest a few trees for energy, and then build a factory. Worker units can also be automated to collect metal from destroyed units and repair your damaged units and buildings automatically while patrolling from one point to the next. The buildings which gather resources do so on a consistent basis – it is impossible to exhaust Total Annihilation's resources, and therefore matches are theoretically infinite.
Unit-building is likewise sped-up. Total Annihilation helped to popularize another one of the RTS genre's necessary interface improvements: the build queue. Instead of consistently clicking on each new unit when the last was completed, now you can select as many of each that you want to have built in one go. If you want multiple units at once, shift-clicking lets you queue five at a time. In this way, you can set up a near-constant stream of new units.
The net effect of these gameplay choices – huge numbers of units, infinite resources, unit-building queues, and automated harvesting and repairs – is such that Total Annihilation feels built around the tactical and strategic aspect of the gameplay, instead of the economic aspects. In another RTS, sending massive armies of mechanical troops to fight and die for tiny amounts of progress would be a waste of resources, and you'd have to develop your economy better on the next try. In Total Annihilation, that's the point. It is in no way a subtle game – just look at its name – but its intense focus on using Real-Time Strategy methods to play out an galactic war worthy of George Lucas is both remarkable, and remarkably successful.