Hamish Todd is a game designer and journalist. His article on Castlevania's medusa heads just made the longlist for the games journalism prize. You can find out about his game, Music of the Spheres, here.

Some of the most fun I've ever had playing videogames has been with the Half-Life series. The games are most famous for their scripted sequences, but there are a lot of other clever pieces of design in them. Today, I'm going to look at a bunch of great Half-Life setpieces all themed around one entity: the barnacle.

The barnacle can do horror, action, and even comedy. It can assist you and puzzle you. To do all that, an object needs to have some pretty fundamental stuff in its design.

This video shows you how Half-Life 2 introduces the barnacle. A lot is communicated by this loud, eye-catching animation (captured by Daniel Holden). It's good because it lets you find out that barnacles are dangerous without you having to endanger yourself -- if you were just walking along and got unwittingly picked up and killed by this unfamiliar thing, it'd feel unfair and time-wasting.

“Many of our scripted sequences were designed to give the player gameplay clues as well as provide moments of sheer terror” ~Ken Birdwell in Half-Life’s post-mortem

So we've been shown by that animation that the barnacles are unmoving ceiling-dwellers that reel up and eat things that touch their thin, dangling tongue. We'll later find out:

If you touch the tongue, you'll be pulled into its mouth unless you kill the barnacle first.

While you're being pulled up, you can look and shoot, but you can't move.

Barnacles have low health.

Barnacles will reel in and try to eat literally anything. This is a double-edged sword: they can go for you and your allies, but also your enemies, which works to your advantage. Unfortunately, AI in Half-Life is pretty complex, so you're unlikely to be able to manipulate a moving enemy into getting caught (it's easier to just shoot it). A barnacle can lend a nice ripple to a shooting arena, though.

Real animals

Half-Life's barnacles have some things in common with real animals. This isn't necessarily because Valve was mimicking nature. It's just that "things which make a videogame enemy effective" and "things which will help an animal survive" can sometimes be the same, so ingenious examples of both will converge.

Jonathan Wojcik alerted me to the fact that the Half-Life barnacles' hunting method of dangling a sticky thread and eating the animals that get stuck to it is a method that has been encountered in nature. It's utilized by some spiders, and by the beautiful “fungus gnat” larvae you see in the video above.

Real barnacles were probably a direct inspiration for the appearance of Half-Life barnacles. Real barnacles have an extremely long, prehensile appendage coming out of them -- but unlike the barnacles we're talking about in this article, that appendage is NOT a tongue!

Half-Life 1

Barnacles have the ability to cordon off danger zones, and that's used well in this area. These barnacles encourage you to take a certain path, and present you with harsh but delicate limitations during a fight. They're also a warning that you're entering a room which is more dangerous than it looks...

In the middle of this picture, on top of the box, you can see some shotgun shells which you'll instantly want to jump to when you enter the room (going down via the sloping path is obviously an unappealing option). But when you get onto the box, the wood you're standing on collapses, leaving you no way back out of the room -- and then a pair of aliens teleport onto the slopes and start running up and down them, shooting at you! It's a uniquely constrained battle, and it gets particularly crazy if you happen to run into a barnacle and have to continue shooting while ascending.





Here's another setpiece. This screenshot was taken while looking directly up. The four greyish lines coming out of the ceiling are barnacle tongues hanging around you.

Barnacles don't move, so they're easy to shoot at -- unless YOU are moving in some complex way. In this scene, the player is standing on a platform that is descending and rotating, a mathematically interesting movement that creates a cage. Aim at a barnacle, and your cross-hairs will curve around and move outward due to the motion.

It is possible for you to pull a lever and stop the platform moving. The barnacles here gently encourage you to start making use of that lever.

Another room, another way of using barnacles. When a barnacle catches you it pulls you up, which is pretty bad because you're going toward its mouth -- but it's not so bad if it means being pulled away from the mouth of something else!

In this water tank, you'll fight the first "ichthyosaur," an underwater enemy [not pictured]. There are half a dozen barnacles dangling their tongues in the water. During the underwater fight, while fleeing the aquatic creature, you might touch a barnacle tongue either accidentally or on purpose. Being hoisted out of the attacking ichthyosaur's reach by a barnacle will provide a strange kind of momentary relief. The ceiling is quite high, giving you lots of time to turn around and smack the barnacle before it tries to eat you.

This is our final thing from the original Half-Life. This setpiece again shows how the immobility of the barnacle makes it useful. Note, by the way, that an unmoving enemy in a game is usually called an "obstacle" -- one example would be bottomless pits in platformers. Barnacles basically do the same jobs as bottomless pits: they carve out "areas that you don't want to go." Barnacles are much neater in 3D than pits though, as the above diagram of the "treadmill" setpiece shows us.

FYI: jumping onto a treadmill is a jarring and stressful experience, because of the abrupt changes in sideways and forward velocity. In Half-Life, and in real life, you're likely to fall off the treadmill if you're not a little smart about it. You do not want to fall off a treadmill! If you fall off one in real life, the floor will punish you painfully. If you fall off one in Half-Life, these barnacles do the same thing.

Think about pits again -- imagine if there were no barnacles here, but instead there were pits on the ground that you'd fall into if you came off the treadmill. It would have the same effect, but the pits would have to take up a lot of floorspace. The barnacle performs the same function in an efficiently smaller space.

Before we look to the Half-Life sequels, here's a scene from Blue Shift. One of the cutest aspects of the barnacle is how you can use them as a way of elevating yourself to a higher place, if you remember to kill them when you get to the top. Several setpieces from different Half-Life series entries use this. The above picture is my favorite example of an "elevator-barnacle."

The player has to cross this gap, but they can't jump the whole thing. They have to hurl themselves at the tongue of the barnacle, which will then hoist them up so they can dismount on the other end.

Why is this the best elevator-barnacle? I like that it involves a jump arc. Also, suppose we made the gap five times wider -- the strategy of jumping toward the tongue would still work, provided the tongue was long enough. We could make the gap five hundred times wider if we wanted!

Now notice the pipe on the left. You actually don't have to use the barnacle to get across this room -- you can just walk across that pipe to get to the same place... although doing so is more roundabout and way more boring. Why did the designers add an extra boring solution to this puzzle? Well, it’s there in case the player (reasonably) assumes they just have to kill the barnacle, and does so... rendering themselves incapable of safely jumping the gap.

Some pretty cool things there in Half-Life 1 then. Now the sequels!

Half-Life 2

Barnacles can be hard to notice. Their body is hidden on the ceiling (nobody ever looks at the ceiling), and they have no movement for you to look out for. And their "danger zone" is so thin. Barnacles can consistently take you by surprise.

As you'd expect for a surprising enemy, there are lots of barnacle-infused horror scenes, but I want to show you the scene in the picture above, which is from Half-Life 2's hovercraft sequence. Look at the thin lines below the bridge -- they're actually barnacle tongues!

Sorry if you have to squint at that screenshot, but that is actually part of the point of these particular barnacles. The player drives under these bridges with great speed, so faint lines in the distance don't stay in the distance for long. This leads to a particularly jarring but funny situation where you can be leisurely speeding along, and suddenly you're suspended in midair while beneath you your hovercraft pootles to a halt. It's a stern warning to reckless drivers, and a novel piece of technology: things aren't usually able to haul you out of your vehicle in driving games!



Here's a great scene from Half-Life 2. The barnacle feature being played with here is "barnacles can be manipulated into moving things around for you." But there's quite a lot more to be said about this corridor.

The scene is a puzzle: you want to get to the exit. But to get through the room, you have to kill loads of barnacles. Shooting every barnacle individually is a tedious waste of ammo since you only have a pistol. However, there is an explosive barrel around. The “solution” is to carry the explosive barrel to a barnacle, let them pick it up, wait until it's been pulled right up close to the ceiling, then shoot the barrel to blow it up. This takes out almost every barnacle in the room simultaneously with only two bullets. It's a good puzzle. It gets you to be resourceful and imaginative. It's made more fun by some satisfying animations and cool physics. But the beauty is in the polish.

There's a specific game design tool on show in this scene: an "antepiece." Let's say you're making a game containing a very challenging setpiece, and it requires awareness of some specific pieces of information. One thing game designers can do to help things along is to have a very brief encounter just before the challenging setpiece which clarifies some possibilities within the game. That's what I call an antepiece (a portmanteau of "antechamber" and "setpiece"), and you can find some clear examples of them in Portal, Super Mario Bros., and The Mighty Jill Off. Antepieces are cool because they reward players for being attentive, and help them get through games quickly and fluently.

This is the exploding-barrel-barnacle puzzle's antepiece. Before you enter the barnacle-filled room, you have to move down this hallway. Your path into the hallway is blocked by three [non-explosive] barrels. To get through you must move a barrel, either by pushing it or carrying it. Wherever you place it, it will roll down the slope in front of you and almost certainly get picked up by one of the two barnacles you can see at the end of the hall. This clarifies three important facts for the player: 1) you can move barrels 2) the floor here is slippery, and 3) barnacles will try to eat barrels. The two barnacles are easy to avoid, so there's zero challenge in this small place. It contributes to the game purely by helping you with the confrontation around the corner.

One last clever aspect of the exploding-barrel-barnacle room is the second slope, just before the huge barnacle crowd. It makes things both easier and harder. Easier because you can just put the explosive barrel onto the slope and it'll roll toward the barnacles on its own (just like what happened in the antepiece). Harder, because you're at risk of sliding down the slope yourself!

Half-Life 2: Episode 1

This is a terrible picture of a room that explores a clever challenge in a minor way. You can see some dangerous barnacle tongues dangling down here, but not the vulnerable barnacle bodies. This shows you how a barnacle can hide. It's the only game enemy I've ever seen hide behind a ceiling! Here's a crude diagram that will hopefully help you understand the challenge:





There are a couple of ways of killing the hidden barnacles. I'm not so interested in them; what I want to talk about is the structure that's built up around these barnacles.

This setup presents a problem for the designers: if us players were just walking along and we came up against a barnacle placed in this irritating way, it might feel unfair. We might wonder “How come the holes in the ceiling convenience my enemies so much?” “How come they get to use this separate upper area I have no access to?” So the designers took steps to make sure it felt fair.

You actually enter the lower floor by breaking through the ceiling yourself. In other words, you got into this room in the same way the barnacles are getting their tongues into it -- so you have less right to feel annoyed at their advantage.





When you've completed this area, you go up a staircase and get to strut, triumphant and risk-free, through the upper room. You can gloat at the barnacles you killed, finish off the ones you missed, and pick up a supply crate or two that you find near them. Most importantly, you are finally given access to this location that was so annoyingly advantageous to your enemies.

Half-Life 2: Episode 2

This is a wordless piece of slapstick comedy. The feature on show here is "barnacles can take objects out of your hands."

We see a machine that requires a cog. We go and get the cog. When we find the cog, we run excitedly back to the machine ... with the cog blocking our view. While we're away, a barnacle tongue has been scripted to descend over the machine. We're not looking where we're going, so it's able to snatch our treasure out of our hands!

When an object is snatched away from you, it doesn't hurt you. It's not a "hostile" act -- it's just a little "mischievous." And when showing off a "mischievous" feature, the best structure to set up around it is the structure of a joke.

A joke is more funny if you have some reason to expect the punchline. And an event in a game is more fair if you have some reason to expect it. Valve's people know this: note that there's another barnacle tongue dangling over the place where the cog is picked up. This implants in your head the idea that you should watch out for barnacles, so it doesn't feel cheap when the new one comes along.

I should apologize for part of this -- I know that a joke is definitely less funny when someone explains it to you. Let's round this off!

My personal assessments

I've described about a quarter of the barnacle encounters that appear in the Half-Life games. They're not all winners: designers sometimes get lazy and just bung a few barnacles into a corridor to make you tiptoe around them. Some exist purely to make a room look more scary or challenging than it really is. So barnacles can be mishandled. But they're still really a cleverly-designed enemy.

Barnacles are fundamentally a great big bluff -- they're actually very unlikely to damage you. When you get caught on a tongue, you're slowly drawn toward a pair of ravenously clamping, sharp jaws. But if you face toward them, you can take them out with a single whack of your crowbar (which never runs out of ammo). At heart, they're "toothless"... and yet, all players will try to avoid barnacle tongues, because those jaws are just terrifying.





Valve is successfully having its cake and eating it too with the barnacle. They've made an entity that you want to avoid, and can therefore be used to make tense, interesting challenges. But it'll seldom waste anyone's time, because it won’t usually kill them.

Barnacles are extremely simple: they're just a line going through 3D space. They make no movements, no decisions. They're sensitive to no contexts. There's no timing to their behavior. Some videogame entities need to be put in specially-set-up environments, but you can always use a barnacle (so long as there's a ceiling in the current environment).

Complex enemies are hard to introduce to players. Barnacles are effortless to introduce. Once you've been introduced to them, the level designers can combine them with other things, like the ichthyosaur, the explosive barrel, the spiral walkway, or the other enemies. It is in combinations of objects that the intellects of designers can shine, because combinations are allowed to be more complicated than enemy behaviors can be on their own.

I see the barnacle as a feat of engineering, rather than a feat of artistry. It's not designed to be in one place, present one challenge, elicit one emotion. It's a tool, to be used to construct any situation you like.

Valve is famous for its design processes. Everyone's voice gets heard, and everybody happily works hard for one another's benefit. It's a little utopia. And in all these moments I've spoken about, I feel the harmony of that process comes through in the game: the barnacle was a tool made by one person, as a gift to a bunch of other people. And it provided great help to those people, in all their varied endeavors.