Let's list off some enemy type tropes you can encounter in a video game, shall we?
- Snipers! (enemies who avoid you and attack from a great distance)
- Juggernauts! (slow and hard to kill)
- Turrets! (immobile enemies with a lot of health)
- Chargers! (move quickly towards you, melee attack, don't turn fast)
- Swarmers! (low health, low damage, attack in large groups)
- Small fry! (easy to defeat, medium health/damage, attack in small groups or individually)
- Teleport spammers! (enemies who spam teleports)
That's a lot of cool ideas! So when you're making a game, you should try to use a lot of those to make sure you've got variety, right?
If that sounded good, you need to take a deep breath and think about why you're making decisions. Why should the players fight juggernauts instead of snipers? Why put in a horde of swarmers? And should your decisions be influenced purely by an abstract desire for variety?
In Portal, there's only one kind of enemy - turrets! There are other obstacles as well, but all obstacles are chosen because they're needed for whatever the puzzle in question is. In other words, you never see a situation where, purely to increase difficulty without changing the approach required, four turrets are used instead of three. Rather, four turrets are used because it makes it impossible to approach from the rear, requiring a different strategy.
This quality - that obstacles are chosen based on the puzzle the designer is constructing, rather than simply how much effort it should take - is, in my opinion, desirable. It translates to non-puzzle games quite well, also:
- Choosing what obstacles a player faces must be guided by what experience the player is supposed to have. This choice should never be solely based on how long the player should spend or how difficult it should be.
Of course, the exception is when the experience you're trying to build purely flows from the difficulty the player is overcoming.
In Red Faction: Armageddon, the variety of opponents feels totally forced. Enemies come in a wide variety of species (I think?) and as far as I can tell, the only reason for this is that they needed more difficult enemies so that battles could be longer without being boring as hell - which brings to our attention the fact that the battles aren't very fun to begin with.
It's really bothersome because I honestly have no clue what these aliens are supposed to be. Are they a covenant of different Martian races? If so, why are they working together (especially when they seem to be non-sentient)? Are they mutated humans? If so, why do they come in so many forms? Why do living beings have laser cannons and plasma bombs? Ultimately, the answer to all of these questions is the same as the answer to the question of why they've got Snipers, Juggernauts, Turrets, Small fry, and the rest:
- Because the game would be boring otherwise.
Variety should be a secondary concern. It should flow from the other decisions you make.
I've included my thoughts on enemy variety in some games:
- When Halo did this they owned it and justified it in the backstory. It felt a little forced - "oh, now I'm fighting elites!" - but it was justified by the fact that the Covenant was literally stratified and composed of differing races with differing abilities. Still, feels like forced variety.
- In Red Faction: Armageddon, every battle is just about the same - there are a bunch of small fry, some heavier enemies, a sniper or two, and sometimes a support tower that buffs everyone. Battles feel formulaic and nothing about them is ever driven by the surrounding events in the game. The only effect on my experience of having different kinds of enemies is that in each battle, I have to take out the small fry with my assault rifle, medium enemies with my magic powers, snipers with my beam rifle, and heavier enemies with my heavier weapon. There's never a reason for them to be there. Such forced variety!
- Mirror's Edge had different types of enemies, but they felt like people employed by the same few organizations - the jump from cops to SWAT officers felt like I was stepping up to a harder difficulty, but when the cops started carrying shotguns instead of pistols it was just different people in the same organization - and I tried to avoid them either way. The step from cops to SWAT officers also made sense in the context of the story, as the police force started realizing I was a threat and as politics happened.
- In Valkyrie Chronicles, variety in types of soldiers is a central mechanic and part of the game, so it's totally justified and doesn't feel jarring - and it doesn't feel jarring because this kind of variety is used well. There's no unilaterally harder enemy type to fight, with the possible exception of tanks. Varying difficulty is provided by changing the size of the forces involved, and by changing the conditions and terrain you're fighting in.
- In Prince of Persia: Sands of Time, it absolutely feels like there's a sliding scale of difficulty and enemies are being picked from it purely according to how hard it should be. Forced variety!
- In Portal, there's only one kind of enemy - turrets! There are other obstacles as well, but all obstacles are chosen because they're needed for whatever the puzzle in question is. In other words, you never see a situation where, purely to increase difficulty without changing the approach required, four turrets are used instead of three. Rather, four turrets are used because it makes it impossible to approach from the rear, requiring a different strategy. No forced variety, not even in numbers!
- Fighting games take extensive care to balance the available characters, so that - if you're controlling for personal skill - you don't choose a character simply because they're better or worse. There is variety, but never the feeling that you're facing an opponent purely because they're harder than the last.