In Orbit, we set out from the beginning with a focus on "differences in kind". Rather than focusing on the boring game of one-upmanship typical of most Tower Defense games, in Orbit we gave each enemy a specific strength and weakness.
Hard Shelled vs Soft Skinned:
Enemies in Orbit are broken up into two different types; Hard Shelled and Soft Skinned enemies. Hard Shelled enemies are shattered by Railgun Satellite while Soft Skinned enemies are fried by Laser Satellites. It should also be noted that Railgun shells pass almost harmlessly through the jelly-like bodies of Soft Skinned enemies while the bolts emitted by Laser Satellites do little against the tough exoskeletons of Hard Shelled enemies. Both Hard Shelled enemies and Soft Skinned enemies are vulnerable to the explosive power of Missile Satellites, although missiles do less damage than Lasers and Railguns. The most skilled satellite commanders will learn which enemies are weak against which type of satellite and exploit their weaknesses accordingly.
All powerful in their own right:
Even the very first enemies you encounter are worthy opponents, having more health than some of the later enemies you encounter. As the Waves progress, more types of aliens will approach the planet and each has a strength that you should be wary of; Scouts that rush at your satellites at great speed, Phases that slip past your shields, Shades that are invisible to your satellites, and Bursting Jellies that detonate on contact with your satellites in a spray of corrosive acid. There are a number of enemies in Orbit which I'll be posting about in detail in the near future. For now, it suffices to say that there aren't any enemies that are simply "upgrades" of weaker enemies, and this leads to a more compelling gameplay experience overall.