Anyone who has played a video game in the last decade or so has probably heard of Achievements - in-game rewards bestowed upon a player for playing a game well. Whether that is completing the game, collecting all its collectables, solving side missions, discovering secrets, beating a set time or "killing 50 zombies on hard difficulty in under one minute using a rifle in the town square blindfolded on a Wednesday afternoon in September when it's raining" (you get the idea).

Each of our games on Steam make use of Steam Achievements, a rewards system specific to the Steam platform which not only rewards players for performing such tasks, but allows them to compare their progress with others, be it their friends or the entire gaming community. Achievements add another dimension to gameplay. Designed correctly, they can extend the life of a game beyond its completion by adding extra challenges that can themselves sometimes feel like a mini-game within the main game. They can also be incredibly addictive - for some gamers, completing a game is merely a journey to gain the achievements, and gaining all the achievements (or "100%"ing the game) is the primary goal.

We've all earned many an achievement through gaming without really paying attention to how that achievement had been thought out, and in doing so we've generally taken for granted that the level of difficulty matched the reward, and that we'd played the game well enough to deserve it. But what about designing achievements and implementing them into a game so that their difficultly levels are just right? Having done this for our own games, here's what we've learned:

Most of the time we employ one rule when coming up with names for achievements: idioms are your friend! Idioms are popular sayings with figurative meanings. Taken as literal phrases on their own they don't make much sense, but they've somehow been entrenched into language such that everyone instantly recognises them. For example, in The Food Run, we have an achievement called Have Your Cake And Eat It which is awarded when the player has eaten 5,000 pieces of food, including cake. The reason for awarding the achievement doesn't actually match the idiomatic meaning for Have Your Cake And Eat It (which means you can't have two incompatible things that are both to your advantage). But that doesn't matter - it's a phrase people recognise. It means you've eaten a lot of cake. And a picture of a cake comes up when the achievement is awarded, so gamers understand it.

These types of achievements represent actions such as completing a level or completing the game. They can be easily split down to give the player something extra for getting to various points - for example completing a single level, completing half of the game, or getting 100% on the game. Every game has them and we think they're essential - when comparing achievements with your friends, often the first thing you want to do is see whether they've completed a particular game that you have, and vice versa. We try to include extremely simple achievements to give players some instant reward for playing - e.g. complete the first level out of 100. This might only take 30 seconds, but the idea is that being awarded such an achievement makes players feel they're playing well straight away, and hence they are more likely to keep playing!

We've all seen these. Kill 100 zombies. Kill 200 zombies. Kill 25,000 zombies. They tend to pop up during normal gameplay (you're here to kill zombies anyway). The tricky thing we found with these types of achievements was getting the numbers right. We don't want them to all be awarded during a normal completion of the game, but we also don't want players to complete the game and find they are only 10% of the way to a progress achievement, because they're not going to want to complete it another nine times to get it. We found the best bet here was to set the levels such that they're more than halfway to the top progress achievement after one run through of the game. This gives a bit more playability without the player feeling they are nowhere near obtaining the final achievements.

As well as including progress achievements for performing the main aim of the game - i.e. eating food in The Food Run - we also added some lesser achievements for performing more mundane tasks, such as pushing crates, hitting stop signs and collecting direction arrows. These are all generally earned during normal completion of the game anyway, but they add a bit extra. We created these lesser achievements as individual ones, whereas for eating food we added multiple, tiered achievements.

Some achievements can really add an extra dimension to a game, and often without the need to change the mechanics of the game itself. These take the form of challenges that usually wouldn't be necessary as part of normal gameplay, but provide something else to aim for, a bit like a mini game within the main game.

We used these to good effect in The Food Run - we added three achievements to the Steam version that required the player to collect every one of a given item on a particular level, even though collection of those items was not necessary to complete the level. In some cases, it even prevented the completion of the level - but that didn't matter if the player was going for the achievement having already completed it earlier.

We've received some good feedback about these types of achievements, so we've taken this on board and will add more of them in future games. Players feel they're getting more for their money if they complete a game, but there's still more fun to be had.

We did learn one harsh lesson when it comes to designing achievements and releasing them in a game: you need to get them right first time! A couple of years after the release of On A Roll 3D on Steam, we naively considered adding some more achievements in a free update give players more to aim for. Thankfully we ran a poll on the game's forums first, and quickly discovered that it was a very unpopular idea! Players who had gained 100% of the achievements weren't too happy about losing their 100% status due to the addition of new achievements. So, we didn't add them, and in future we'll make sure we add everything on initial release and keep it that way.