One of my friends lately asked me whether he should play Spelunky, and I said yes, unequivocally. "Is it fun?" he asked me, and I said, well, not exactly. Derek Yu and Mossmouth's Spelunky has been a fan favorite since its initial launch on PC in 2009, something of an homage to games like La Mulana. But since its robust release on Xbox Live Arcade -- featuring a co-op mode -- it's been lighting up appetites and imaginations on an entirely new scale. As a platform-oriented, more modern take on the roguelike genre, its most visible characteristics are its randomly-generated levels and its relatively severe difficulty. Progress is hard-won and as such is extremely gratfiying; that quality has earned it some comparisons to Demon's Souls, Dark Souls and its ilk among fans, but that'd be an imperfect parallel: while those challenging games reward players for memorizing their brutal worlds and using their learnings to inch a step or two further each time, Spelunky victories always feel like the perfect cocktail of applied skill and pure, random luck. Spelunky lends itself to being highly session-based, too -- most players won't spend more than five or ten minutes at a turn (plenty even less!) before needing to start again. Yet gradual progress can be won by preserving and delivering precious, even necessary resources to the mysterious Tunnel Man at the end of each set of four stages; this will usually take several trips, but the payoff is being able to play from the beginning of a new level, rather than back at the beginning. Of course, the objective is to gather treasure: Loot gathered can be spent in shops to gain much-needed supplies, like bombs to penetrate walls or ropes to negotiate significant climbs. Shopkeepers also sell specialty weapons and power-ups that can provide a rare sense of empowerment and advantage when all seems lost. The player's health is both paramount and scarce; every injury, whether that's falling from a height or making contact with an insidious spider, costs hearts, and players only get four. But grabbing "damsels" -- distressed travelers hidden in Spelunky's hazardous zones -- and successfully carrying them to a level's exit will restore one. The result is a surprisingly complex, well-balanced and hooky experience: Players leverage skill against luck, and gamble progress against greed and the desire for power. Time is of the essence, too: Spend too long in a stage and an enormous ghost arrives that can be nearly impossible to evade. Many uniquely wicked and challenging games have caused designers and players to question the widely-accepted supremacy of "fun" as games' holy grail. Spelunky may be another example of why imminently-attainable and generous rewards aren't always the ideal recipe for design success. More than that, its incredible balance and character makes it one of those games that shows steep challenge and good old-fashioned fun aren't mutually exclusive. Even players that don't like steeply-difficult games find something to love about Spelunky -- here's why it works so well. You can trust the game. Spelunky's randomly-generated levels aren't blindly abusive; its visual language communicates clearly with the player so he or she always knows enough about what to expect. Just enough of the geography in each level is predictable that the player can make educated guesses about where to seek big rewards, or where to find the exit. The hazardous jungle occasionally plays host to a bottomless lake; the mines occasionally hide dangerous snake pits. But the game always warns you when these special conditions occur, so they become exciting new challenges with known parameters, not sudden unfair twists. More importantly, the game is balanced so that no matter what, a player getting stuck in a stage with no chance of progress is extremely rare. Ropes and bombs are generally essential to navigation, but players that have run out of both will often find it's possible to complete an area anyway. This gives the items just the right amount of weight: They feel valuable, but the game doesn't feel absolutely pitiless. Risk and reward are in perfect harmony. There are big payoffs to be had for taking crazy risks, while cautious players will make slow, moderate gains. This leaves room for all players to customize their experience in accordance with their skill level and affinity for thrillseeking -- a sense of control and ownership is key to player gratification, so long as it remains in good balance with friction from the environment. In Spelunky, players will frequently find shopkeepers selling pricey advantages; their selection and its cost will be randomly-generated. While essential ropes and bombs are often on sale at attainable cost, there are bigger prizes to be had. A pair of climbing gloves will preserve one's rope stash, for example, while a pair of spike shoes allows players to stomp big baddies that might otherwise be too formidable to take on without bombs. Patient players might have amassed enough loot to purchase a perk or two --but for daring players who'd like to play the thief, stealing is possible. Stealing enrages the shopkeeper, who'll go on a lethal rampage in response to any ill doings in his area, including an "accidental" bombing. But it is possible to kill or evade him, and players who succeed can collect the lot of his wares -- sometimes a delightful and well-earned embarrassment of riches that feel like getting away with murder. Of course, shopkeepers that appear in later levels will still be angry and dangerous, but it's pretty much worth it. Players can also sacrifice the "damsels" they rescue to a mysterious dark god called Kali, should they chance across one of her altars. They lose out on a heart, but the reward might be better. Rare hidden idols occasionally bring big cash payoffs for players that can hang onto them -- and survive the traps they trigger. Occasionally Spelunky even gives you a level plunged in darkness with a torch to carry -- intimidating but also exciting, since these levels contain glowing scarabs worth tons of money for those who can survive. This complex ecosystem of exciting and fair trade-offs makes Spelunky a continually-personal and intellectual exercise, just as it is a test of twitch and timing. This is very important to getting players to tolerate continual challenge in a world where much is randomly-generated. You can expect the unexpected. Spelunky is packed with surprises, and even players who've dug deeply into the game and who play it often are guaranteed to eventually stumble across something they've never seen before, whether that's a buried secret, a rare item or an unlockable character, hidden in an ominous tomb. This keeps the sense of adventure alive. The game's accompanied by a sort ofdigital lorebook that expands with each new encounter, and triggering new entries is often fun in a way that defies explanation. The best thing about an environment where players are always ready to be surprised is that they become more experimental and curious -- and in a game where risk can bring vast peril, it feels brave and fun to test and try new things. It has a sense of whimsy and humor. Praising Spelunky's playful visual style only scratches the surface of its appeal. Its broad selection of playable characters all feel the same to play, but they're each distinct enough to be meaningful, and their unique and even friendly looks encourage players not to take the game's hostile world too seriously. In another excellent twist, players can choose what kind of "damsels" they rescue: those tired of saving the helpless lady can save a helpless, pretty man. Or a pug dog that whines balefully from within whatever prison the game has chosen for it. Or a mix of all three, an amusing subversion of a common trope. The game's tone is simple enough to recall the pleasures of childhood or of old-school games, but since Spelunky itself has so much balance and depth, it feels like a nice contrast. The co-op is really social. Because so much of playing Spelunky is personal, determined by an individualistic approach to challenges and judging risk-reward situations, its co-op mode necessitates a couple of players who want to talk and strategize together. The lead player can't run off on his or her own treasure hunt without dooming the other; how to tackle every bit and spot, how to distribute, use and share resources needs well-rehearsed harmony -- or at least a good planning chat. Players are able to carry one another, and those who die can follow the survivor as a ghost that can blow key items into their pal's reach; those who live will get a chance to save their partner from the tomb. The good news is that if you and your co-op buddy are poor at collaborating or have a sense of mischief, it's still fun and funny. Co-op for such a deep and individualistic game seems like the kind of thing it'd be hard to pull off well, but Spelunky manages. Back to my friend who asked me about Spelunky and I told him it wasn't exactly fun. "You have to be patient," I said. "There's a lot of dying and learning your way in." But then I realized I was totally smiling. "It's fun, though, too," I told him. "Super fun."