Most of the offense-oriented roles we’ve covered have been pretty fast as a rule; swarmers have to move quickly and kamikazes have to be able to keep up with their targets. By definition, they’re usually close range as well. It stands to reason that some monsters, however, don’t have to move around much and are still dangerous from a distance. Turrets are one of those monster roles that isn’t very mobile (though they don’t have to be immobile) and usually attacks from a large distance.
The Vore from the first Quake is one such monster. Of course, the word turret implies something immobile, but the Vore can actually move around. It’s pretty slow, however, and is usually placed far away, across terrain it can’t traverse (across a gap, on a ledge, etc.). Because it’s so suited to long-range, its attack has to be able to hit at those ranges; thus, it has a projectile that homes in on you (famously so, since it’s capable of hunting you around corners) that’s hard to avoid and does a pretty decent amount of damage for what it is. It also has the second-highest health in the game, so it won’t die particularly quickly.
The first problem you run into when designing a turret is what kind of attack to give it. Because they’re usually placed away from your target, projectiles don’t usually have a good chance of hitting. The Vore gets around this by having a famously aggressive seeking projectile. Alternatively, you could use a hitscan attack; an attack with an instant travel-time, like the bullet-based weapons in many older games. Hitscan attacks on turrets do have problems (and indeed, their own class), but I will go into that in detail later. There’s no hard-and-fast rule for these, you’ll just have to plan out how your turret will work and be careful how to design and balance its attack.
The Icarus from Quake 2 was another decent turret, with a twist. It had a quick-moving projectile attack (quick blaster burst), and could take a bit of damage, but the major difference was that it hovered in the air, making it a bit more mobile than the Vore was. Usually they were placed high up where the player wouldn’t expect, also, and it was still pretty slow so most of the time its added mobility was just so it got a better firing angle on you. Its rapid-fire, fast-travel projectile attack also helped it actually hurt the player from far away. It didn’t have any hitscan attacks or homing projectiles, but its attack was so quick that you have a good chance of taking damage from it. They were usually encountered in small groups of two or three in later maps, as well.
Turrets benefit quite a bit from good positioning. If they aren’t placed well, they might not be able to hurt the player, or they can be avoided pretty easily. However, if your turret can fly, like the Icarus, it can reposition itself as it needs. The only major issue with that is that you might not be able to restrict it to a particular area (though if you need it to reposition, you might not want to).
One more variant of glass cannon is the sniper, and though there are many kinds (Deadeye from Binary Domain, Combine snipers in Half-Life 2, etc.) the best example I can think of is the Jackal Sniper from Halo 2. Like all glass cannons, they died very quickly and had very-damaging attacks. The difference between more ordinary glass cannons and sniper-types is that sniper-types have long-range attacks that are USUALLY (but not necessarily) hitscan. The Jackal Sniper had a Beam Rifle that was capable of wiping out your shields in a single shot, and was very difficult to avoid because the attack instantly hits, so you have to depend on its random chance to miss.
The major downside to sniper-types is that they’re even more of a luck-based challenge than more usual glass cannons. I’ll use an example from a well-known Doom mod; In Ultimate Torment and Torture, there were one or two particular maps that featured Zombie Railgunners and placed them pretty far away. The big problem was that they could hurt you from across the map for a decent chunk of damage every time they hit. In an effort to balance this they’re usually very weak, health-wise, but you still couldn’t easily kill them. Most of their ability to survive is that they’re so far away, and in the case of those Zombie Railgunners they’re impossible to hit with most weapons in the game. Like other glass cannons, they’re very difficult to properly place without being either useless in function or unfair on the player. The upshot is that they can work very well in accomplishing the job turrets are made for, if they’re well-used and well-designed.
Speaking of which, the main reason to use turrets is as area-denial. Sort of like a support, turrets are designed to back up encounters with other monsters, but they do so by reducing the area in which the player can operate. If he does go into the turret’s line of sight while fighting other monsters, he risks taking even more damage than he would otherwise. Because of this, they should be restricted to a certain spot so they can keep an eye on that area (definition of a turret), and they have to do enough damage that the player doesn’t want to draw its attention. Conversely, they should be tanky enough (or far enough, especially in the case of snipers) that they can’t be easily wiped out by the player. You might think flying turrets go against this, but not necessarily; they still might be denying an area, but that area itself is shifting, depending on how you design the area itself and how that flying turret moves. Different flying turrets can move in different ways to cover an area more effectively. For example, The Icarus constantly moves towards you (it’s the standard monster AI for all the early Id Software games), but the Heavy Hovertank from Quake 4 shifts around its position randomly to try to get better angles on you.
Turrets can also work well combined with other roles. The Ravager from Red Faction: Armageddon is a good swarmer/turret; they bounce around back and forth between walls and ledges, doing ever-increasing damage to force the player into or away from an area. Like other swarmers, if you kill one, it reduces the threat, but the effect isn’t completely gone. Depending on the design, turrets can also be tanks, as well; since they do so much to shape the area the player can fight in, they demand some attention (The Vore does this). And fitting the purpose of these articles in general, turrets work well when placed in tandem with other roles. Imagine an arena with some limited cover from a turret, with a tank and swarmers being the real challenge of the encounter. The tank takes your attention while the swarmers try to deal the damage, more effectively because the turret is limiting your movement.
Battlefield control is one of those elements of design that map-designers like to have a lot of flexibility with. Turrets let them design those more interesting encounters, especially in games where the player is especially mobile. Doom 2 and every (single-player) Quake game have these to balance out how capable the player is. We can use these to make the game more challenging, and more fun for the player.