Remember when Dungeon Keeper was good?

In Terry Pratchett's sublime parody of Faust, simply titled Eric, the Demon King of Hell Astfgl adopts a radical new approach to facilitating the suffering of the eternally damned. Rather than a chaotic nightmare of fire and brimstone where the air is filled with the screams of tortured souls, Asftgl's vision of hell is a bureaucratic heaven, one in which the demons wear cheerful name badges with "How May I Help You?" scribbled on them, and where, before a soul embarks upon a punishment of Sisyphean Labour, they must first be read all 1,440 volumes of the Unhealthy and Unsafety Regulations governing the Lifting and Moving of Large Objects.

Astfgl's idea is that insufferable boredom is a far better method of torturing a disembodied soul than sticking a red-hot poker up its intangible behind, and he is probably right. But Astfgl makes one fatal miscalculation - corporeal torment is as much for the sake of the demons inflicting it as it is the souls receiving it. Running a successful enterprise isn't simply about efficiency. Job satisfaction is also a crucial factor, and you aren't going to keep a bunch of sadistic demons happy by having them read out tomes of protocol until the end of time.

Whenever I play Dungeon Keeper, I always bear in mind the folly of Astfgl. Dungeon Keeper tasks you with conquering a disgustingly lovely fantasy realm by building an efficient subterranean machine that doles out death and destruction. But at the same time, the workforce you employ has needs that must be catered for, and neglecting those desires could result in your minions literally breaking your Heart.

Duality runs through the core of Dungeon Keeper like a vein of glittering gems, present even in its wickedly simple slogan. "Evil is good." Over the years I've seen it referred to separately as an RTS and a management game. But the entire point of Dungeon Keeper is that it is both. It's a genre mash-up with a formula so good that its many imitators, from Evil Genius to last year's War for the Overworld, have never succeeded in eclipsing it. Even its own sequel couldn't match the intoxicating blend present in that first serving - the last original work from Bullfrog Productions.

The broader structure of Dungeon Keeper is that of a traditional RTS. Progressing through a linear campaign, each mission requires you to build a base, harvest resources, recruit units, and ultimately defeat the enemy force, be it a bunch of sickeningly righteous Heroes, or a rival Keeper. Yet unlike the majority of RTS games both then and now, Dungeon Keeper only allows the player indirect control of their units.

Initially, this is straightforward enough. Your starting creatures, the chittering, skittering Imps, are slavishly loyal to their keeper, mining out rooms, depositing gold in your treasury, and claiming territory in your name without ever complaining. Meanwhile, early units such as beetles and flies, attracted through a portal by your dungeon's amenities, only have very basic needs such as food and shelter, and as such are easily pleased.

Soon though, tougher adversaries require more elaborate dungeons which attract more powerful but also more demanding creatures. Despite being almost twenty years old now, Dungeon Keeper's use of AI behaviour-states to create personable units remains a landmark design that only handful of games have come close to equalling. Warlocks possess powerful magical abilities and keenly research new technologies for your dungeon, but they're also exceptionally snobbish, and will become enraged if a creature whom they deem intellectually inferior enters their library. The corpulent Bile Demons are hardy fighters and talented craftsmen, but require large lairs and hatcheries to keep satisfied, and won't tolerate the presence of an enviously thin skeleton. Then, of course, there's the Horned Reaper, a whirlwind of rage who is as formidable in battle as he is impossible to control. Absolutely everything pisses off a Horned Reaper, not being paid on time, not being fed, not being kept busy. It's like having the world's most entitled celebrity staying at your hotel, simultaneously a privilege and a pain in the arse.

This influx of increasingly exacting creatures is coupled with a gradual layering-up of your power. The types of room available to you shift from attracting and maintaining creatures to enabling you to create new ones. Prisons, for example, let you capture enemies and slowly starve them to death, at which point they'll rise as skeletons. Torture chambers, meanwhile, let you convert enemy creatures to your cause. Even the dead have their uses; burying them in the graveyard lets you turn them into powerful vampires. Yet for each new creature that joins your dungeon, the chance for conflict grows alongside the likelihood of victory. It's degenerate Jenga - the higher you build, the more unstable the foundations become.

What's ingenious about Dungeon Keeper's merging of management and RTS is how is solves the most common problems it those respective genres. The biggest issue with management sims is the lack of a hard end point - they tend to tail off into tedium as you run out of things to build. But Dungeon Keeper always has a specific goal for the player to work towards - the obliteration of the enemy. On the RTS side, Dungeon Keeper prevents players from relying on the age-old "build 'n' rush" strategy, because your units don't just sit around waiting to be commanded. Eventually, something will kick off, be it a missed payment causing a riot, or two creatures fighting because they hate each other. You need to time attacks effectively, use the right units and, if possible, support them with spells.

Admittedly, the game doesn't convey the strategy behind combat particularly effectively. Aesthetically Dungeon Keeper has not aged well - its textures are a muddy mess of browns, blacks and reds, such that it becomes difficult to interpret what's going on when more than a few characters are on-screen. It also runs terribly, giving the same kind of framerate during larger battles that you'd expect from the latest Crytek game.

Mercifully, the vast majority of Dungeon Keeper's personality resides within its sound files. Dungeon Keeper's sound design does a fantastic job of making a strategy game viewed from overhead feel like it's taking place underground. From the eerily soothing pulse of your dungeon heart echoing through its chambers, to the metallic clink, clink, clink of an Imp's pickaxe, be it one of yours, or an opponent's moments before their creature hordes pour into your dungeon. The music further complements this, existing halfway between soundtrack and eerie ambience, again emphasising the game's love of duality.

Easily the highlight of the game's sound design, however, is Richard Ridings' delightfully wicked pre and post-mission summaries. Voicing the Dungeon Mentor, he comments on the idyllic beauty of Dungeon Keeper's fantasy pastiche with a withering disdain that Alan Rickman would be proud of. "The people of Eversmile are plagued only by aching facial muscles, and not Anthrax as we'd hoped." "Cosyton - a hideous sham of a town in which the prosperous citizens have no gripes or moans." It sets the tone perfectly, framing these quaint fantasy realms as sickeningly saccharine abominations, like drinking a jug of melted Mars bars.

Dungeon Keeper has nothing insightful to say about the nature of evil, apart from possibly that being a baddie is bloody hard work. But arch meta-commentary isn't what it aiming for. Dungeon Keeper may be a parody of fantasy, but it is also a fantasy in itself, merely an inverted one. Hence it frames your dark crusade with the same unwavering righteousness as Tolkien does Frodo's in the Lord of the Rings, while painting your enemies with the same simplicity. Perhaps there is something in the notion that evil is simply having an unusual perspective on what is "right", but Dungeon Keeper revels in the idea rather than exploring it.

If Dungeon Keeper has a message, it's one communicated in algorithms rather than words; how to create unique, identifiable characters through a game's mechanics, how to balance AI autonomy with a player's guiding hand, how managing opposing systems can create singular stories and dramatic experiences. What is ostensibly a silly fantasy send-up contains some remarkably intelligent ideas, which give it more in common with Terry Pratchett's work than merely Astfgl's inventive approach to damnation. It may not be Bullfrog's most celebrated game; most would still make the case for Syndicate. But in terms of what we can still learn from it, I'd argue that Dungeon Keeper is their most important.