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This is Game Maker's Toolkit, I'm Mark Brown.

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If there's one game I've played more than
any other, it's Spelunky - a viciously hard

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platformer that takes the side-scrolling thrills
of games like Mario and Castlevania, but snatches

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away your ability to learn the level layouts
and enemy spawns by mixing up the map, every

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time you die.

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But what makes Spelunky so special is that
even though the levels are randomly cobbled

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together, they're always fair and enjoyable.
Even after 1000 runs, I've never stumbled

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upon a crappy level.

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They just don't feel like the product of an
algorithm - which, in some games, spits out

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levels that are messy and unfocused and full
of dead ends. In Spelunky, these procedurally

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made stages are as sharp and satisfying
as the levels in some hand-crafted games.

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And that's because creator Derek Yu struck
the perfect balance between random and authored

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content.

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He explains how he pulled it off in his new
book, titled Spelunky, which I highly recommend

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to anyone interested in design. But, while you're
waiting for it to come in the post, I'm going to use

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some info in the book to explain how a Spelunky
level is half generated, and half designed.

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And then we'll look at why it was so important
for the game to have an infinite supply of

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unique levels.

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Though, first, we need to go back to where
it all began - because while you might think

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of Spelunky as looking like this, it actually
started life as a free, pixel art game, now

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named Spelunky Classic.

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So, every level in Spelunky is generated by
a script that starts with the same basic shape

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and size: 16 rooms in a boxy 4 by 4 grid.

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The first thing the script does is pick a
random room from the top row, and makes this

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the entrance.

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It will then randomly place a room to the
left, right, or beneath this room. This process

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is repeated from room to room - and if the
path hits the edge of the level, it also goes

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down - until we get to the lowest level; when
it tries to go down once more, an exit is

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made.

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Every room on this main path has openings
to the left and right, but rooms where the

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path drops also have exits on the bottom,
while the rooms you drop into are given exits

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at the top.

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What you end up with is a guaranteed path
through the level that you'll always be able

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to get through without using ropes or bombs.
The other rooms are not on the critical path,

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so they may be open, or they may be walled
off depending on their design.

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Next, each room is randomly given a template.
Derek Yu hand crafted a number of room designs,

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with different layouts for rooms where you
drop down, rooms you land in, corridors you

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run through, and rooms not on the critical
path.

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But these templates are not set in stone,
and parts of them are randomly generated.

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Sometimes entire chunks of tiles are plastered
on at random. This means that while you might

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start to see familiar set-ups, they will always
have their own unique quirks.

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Next, a script checks every tile on the map
and rolls a dice to see if it should place

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down a monster, some treasure, or another
object. Again, it's not entirely random - gems

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and crates are more likely to appear in spaces
surrounded by walls, and enemies generally don't spawn

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in cramped spaces. And everything is weighted,
so a level isn't filled with fire frogs or

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crates containing jetpacks.

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Describing his level-making algorithm, Derek
Yu says "This system doesn’t create the

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most natural-looking caves ever, and players
will quickly begin to recognize certain repeating

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landmarks and perhaps even sense that the
levels are generated on a grid. But with enough

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templates and random mutations, there’s
still plenty of variability. More importantly,

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it creates fun and engaging levels that the
player can’t easily get stuck in, something

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much more valuable than realism when it comes
to making an immersive experience.”

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There's still more to a Spelunky level, of
course. Those rooms not on the critical path

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can contain golden idols that set off a boulder
trap, or a sacrificial altar where you can

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trade bodies for helpful items.

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A room can also be a shop where you can buy,
or steal items. “The difficulty of buying

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all the items you want and the consequence
to your score" - your final score is based

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on how much loot you found - "creates a strong
temptation to steal from the shop", says Yu.

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If you do pilfer an item, the shopkeeper will
not only try and kill you with his shotgun,

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but his buddies will appear at the end of
every level for that run, and attack you in

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a mad, unpredictable fury.

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Every level also has a damsel in distress,
which gives you one extra health point.

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While the health pick-ups in other games - be
it a mushroom or a turkey leg or a pizza - are

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activated immediately, Spelunky makes you
carry the damsel to the exit, leading to tricky

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moments where you have to lug multiple items
through an unpredictable maze.

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And finally, there's the ghost. Stick around
in one level for too long and a spectre will

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appear and instantly kill you if it touches
you.

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All of these additions, which are highly authored,
but scattered about the procedurally generated

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worlds, are about making choices and weighing
up your options. Derek Yu says "I wanted to

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force [the player] to make difficult decisions
and experience both the satisfaction of choosing

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correctly and the regret of choosing poorly".

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So do you sacrifice your score by spending
money on the shop? Or do you just rob it and

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deal with the consequences? Do you risk throwing
a damsel down here, or leave your shotgun

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behind? Do you grab the golden idol, and risk
getting smushed by a boulder? And can you

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collect these gems before the ghost appears?

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Those decisions would be far less interesting
to make if you had already played the level

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before and knew the right path to take or
could predict the exact outcome of snatching

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a golden idol. But because you never know
how the stage will be laid out, the game forces

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you to read the situation and make a plan,
before watching it all go hideously wrong.

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And that's all the more painful when you know
you'll be sent back to the very start of the

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game, now with all new levels to tackle. The stakes in this game are super high.

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Plus, not allowing you to master the stages
means you'll need to master the mechanics

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instead. You'll need to understand the quirky
physics of your jump and the tiny reach of

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your whip. You need to learn the unique properties
of each enemy, so you can to predict what

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will happen when you enter their domain. And
you have to learn about the items, and the

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secrets, and the tricks that will keep you
alive for longer.

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So Spelunky proves that an algorithm doesn't
necessarily lead to soulless levels - balancing

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authored and random content can lead to levels
that are satisfying to play. And by removing

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the ability to learn levels - either after
you die or on your second or 1000th playthrough - this

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game encourages you to learn the underlying
mechanics, and makes you read the situation

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carefully before making those very difficult decisions.

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Thank you so much for watching!

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Spelunky is one of my favourite games of all
time so it's great to be able to dig into

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just one aspect of what makes it so special.

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I will no doubt come back to the other bits
- like the emergent gameplay and the crazy

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secrets - in future videos

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