The term terrain, in the context of roleplaying games and wargames, refers to both the ground under the feet of your character, as well as the non-living objects around your character. If your character is in a forest, then the terrain is the forest floor, all its growth and composting leaves, and also the trees and bushes. If your character is in an office building, then the terrain is the carpet or tile floor, walls and windows and doors, and desks and tables. It can often fade into the background because, well, it is the background. However, when it's properly integrated into game play, terrain can transform your battles.
Not coincidentally, transforming battles is exactly what terrain has done in real life for as long as there have battles. There's a bunch of history written about how the land shaped has decided the victor in some wars, or how it's prevented war altogether. Think about the Centrones ambushing Hannibal only once his troops reached a narrow pass, or the Persians secretly flanking the Greeks along Mt. Anopaea in Thermopolyae, on Napoleon's forces being beaten down by the harsh Russian winter. Historically, terrain isn't just the place where the battle takes place, it is the battle.
In a very pragmatic sense, when you set up terrain on your gaming table, you're providing potential havens for cover. A soldier ducking behind a barrier is harder to attack than a soldier standing out in the open. Mechanically, that might mean the soldier has a temporary bonus to defense, or maybe the attacker has a penalty to attack.
This doesn't just add realism, it also adds choices. A common trap some games fall into is that combat starts to feel like people are just standing around motionlessly as they hit each other with clubs. If somebody hits me in real life, I'm likely going to run. However, games often threaten you with Attacks of Opportunity or Desperate Escape saves if you try to move your character out of a threatened space. And anyway, if you run then you're just delaying the inevitable. Your enemy's going to run after you and continue its attack. So characters just stand around, calmly shooting each other in the face until one is out of HP.
Running for cover sounds like a believable action, though. That may be worth the risk of leaving a threatened space, because you end up in a fortified position. Your enemy might also run for cover. Then what do you do? Maybe you could take a few shots from your protected position, let your enemy do the same, and then leap over the barrier and charge in hopes of a surprise attack, or else to drive your foe away.
A simple combat interaction can become surprisingly complex when you incorporate terrain.
I say it can become complex because the mere presence of terrain doesn't mean all players are thinking about what benefit terrain provides. More on that later.
Terrain influences tactics outside of just providing cover. I've played games with people who had played plenty of games on battlefields that were only vaguely defined, and I'll never get tired of witnessing them realise all the new potential that terrain unlocks. It's like seeing new neural synapses (I'm not a neurologist, I'm just making up words) form in real time. A person whose solitary combat tactic used to be "I roll to hit" can gain a whole new vocabulary once well-defined terrain is introduced.
This is especially true, I have found, when a player is forced to change tactics due to terrain. There's debate over how much terrain is too much terrain, but I find that for players new to the concept of an environment they can actually see and interact with, more is better. If you can't stand in one place and just roll to hit 99 times until your enemy finally dies, then you have no choice but to spend a turn moving around or up a building, or placing a character or squad in one building to cover your melee character, or to surround the enemy, or whatever.
The possibilities are so endless that it can change the game. Boarding Actions in Warhammer 40,000 is so different to how 40k usually plays that it's literally considered a unique game mode, with its own rulebook and army restrictions. Mordheim for Warhammer, a game famous for its use of terrain, is such a unique gaming experience that community sites continue to keep it alive even though it's been out of print for decades. Middle-Earth Strategy Battle Game has pages of rules to define how miniatures interact with terrain, ranging from walls and ramparts to doors and windows. Terrain isn't just scenery, it's part of the rules.
I've been mostly speaking about [in game] man-made terrain, but of course terrain also includes natural features of the land. Natural terrain in an adventure game is often as dangerous as the enemy itself. There might be puddles of water, or mud, or lava, overhanging rocks that are given to crumble, carnivorous plants, explosive flowers, spiky cacti, corrosive rainfall, and whatever else you can imagine.
Difficult or dangerous terrain can impose temporary penalties or even direct damage, to characters that are driven into its range.
The tricky thing about difficult and dangerous terrain is that once you tell the players about it, their only logical response is to avoid it, and usually at all costs. The terrain may as well be force field, because no player goes near it and so its effects are never triggered. There are lots of great ways to make difficult and dangerous terrain fun but the most obvious is to force the players to move through it in order to reach their goal. Maybe it's a river of lava cutting the battlefield in half, or a trapped doorway into the room containing the treasure chest, or a computer that's been rigged to strike back when hacked.
Difficult terrain helps make the battlefield come to life.
In addition to being game pieces, terrain is a narrative tool for game designers and game masters. A battlefield with burning houses and still unopened treasure chests tells you something about what happened just before your character or army arrived. A battlefield filled with sarcophagi and statues tells you literally where the combat is taking place. A town that's long since been looted and abandoned tells one story, while temple ruins that have obviously existed for centuries tells another. Terrain is part of the setting, and through context clues it can reveal much about the game world.
Finally, terrain looks good. Playing a miniatures game on a 3D battlefield is fun. It's just really satisfying to break down one more wall between the real world and the imagined one you're playing in.
I don't think the terrain even has to be all that good to look great. I've cut up some cardboard boxes, painted them gray with cheap craft paint, feathered the edges with black for smoke effects, and ended up with a ruined cityscape in an afternoon and about $10. Trust me, most players sublimate your rudimentary crafting skills the moment their imagination takes hold. And seeing a battlefield they have to navigate, using tactics, makes imagination kick in fast.
It can be difficult to integrate terrain into game play, sometimes. It's easy to forget about terrain entirely when the terrain is just painted onto a photo-realistic battle map, or drawn onto a dry erase mat. Even worse, it's nearly impossible to track when you've got no map and are running a game entirely "theatre of the mind."
Here are some tips on bringing 2D terrain to life:
Here are some tips for 1D terrain:
I struggled with terrain for a long time as a game master for roleplaying games. Once I started building my own battlefields for wargames, however, the promise of terrain was finally fulfilled. Craft a ruined building, or a stone monolith, or just a crate or barrel. It'll add a whole new dimension to your game.
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