Sunday, September 11, 2011

The Binding of Isaac Gameplay explained

So, I've obviously been seriously neglecting this dev blog due to crunch time with The Binding of Isaac, but things are currently slowing down due to the game being basically finished minus some more testing and polish so i thought it would be a great time to do my 1st in depth blog post about the design of Isaac.

For the past month we have been heavily testing Isaac with other devs, the response is extremely positive and the majority of testers are fully hooked, but there is one core issue.
The 1st time people play they don't understand that the game was meant to be played multiple times with different results.

I've tried to push the title "Roguelike" around a lot to get across how the game plays, but I'm now realizing that a majority have no clue what that means, they see something that looks like a zelda shooter and right away assume it's going to be a linear experience and then become frustrated by their 1st death when they respawn back at start.

So here is my attempt at explaining the game :) I think I'll call it...

Randomly Generated Content! it's good for you!

The goal with Isaac was simple, I wanted to design a game that would feel totally fresh and new every time you play it. Now on paper this might seem very hard to do, but by pulling from the roguelike formula I was able to do this pretty easily.

On the surface the first thing most people will notice after playing once and dying is that the maps are generated randomly.

Every map consists of 4 core rooms, and between 5-20 non-core rooms.

The 4 core rooms are:

Treasure Room: where you will always find an item
Boss Room: where you fight the boss end exit the map
Shop: where you can buy items
Secret Room: its a secret!

The 4 core rooms will ALWAYS appear in a map, they are the foundation of the games "leveling" system and should be looked at as the goals of each level. Even though these rooms always appear, their contents, enemies, look, design and any "surprises" within them are randomly picked from a slew of alternates.

The non-core rooms are randomly selected from a pre-designed group of 1000+ rooms that are arranged by difficulty and chapter. the majority of these rooms consist of small arena battles that make the bulk of the game play, but they are also peppered with bonus rooms with mini puzzles, prizes, traps and a few surprises here and there.

Aside from these rooms there are also a set of context sensitive rooms:

The Arcade: A room filled with mini games that cost coins (only appears when you have enough money)

The Challenge Room: Fight off waves of enemies for a prize. (only appears when you play well)

Mini boss room: a mini boss appears!
Mystery Room1: it's a secret!
Mystery Room2: it's a secret!

These uncommon rooms are meant to spice up game play and add small alternate goals and or extra risk/reward aspects to the game. as per usual the contents of each of these rooms is always different.

On the surface this is basically the foundation of the games design, but if we go another layer deeper we can see that the contents within the randomly generated core content also have random elements inside of them.

Now, we know that the rooms are chosen randomly from a very large set of rooms I designed, but the contents within each of those rooms also have random elements.

There are well over 50 enemies in Isaac, but each of these enemies have minor random elements:

For example we have this guy, the Mulligan.
Most of the time he looks like the 1st guy, runs away from you if he sees you, has a small % chance of blowing up and shooting bullets out and spawns 2-4 randomly chosen flies when he is killed.

But there is also a smaller % chance he will appear as the 2nd guy, the Mulligoon.
Mulligoons act the same as their brothers accept they always explode into a spray of bullets when they die and also leave a "troll bomb" behind that blows up.

On top of these there is an even smaller % chance that a Mulligan will appear as a "Special" Mulligan and possibly look like the 3rd guy.

Special enemies will be randomly colored, slightly larger then their brothers, do more damage and have more health.

Special enemies will also drop "pickups" when they are killed as an extra reward for the risk of fighting a stronger enemy.


Whenever you clear a room, kill a special enemy or open a chest there is a good % chance pickup items will be spawned.

Core pickup items consist of:

Pennies: you buy items, play games and donate these to hobos.
Hearts: you fill up your life with these
Bombs: you blow up rocks, doors, walls and enemies with these
Keys: you unlock shops, treasure rooms, locked chests and other special things with these.
Less common are the one use pickup items:

Tarot Cards: randomly picked from the 18 cards that exist in the game. each card has a different effect.
Pills: randomly picked from around 12 different effects, both negative and positive. every new game changes what color pill has what effect(much like scrolls and potions in roguelikes.

On top of these, there is also a small % chance that the core pickups will become these:

Nickle: 5 pennies!

Half Heart: :(

Soul Heart: acts as armor

Double bomb: 2 bombs
Troll Bomb: :(
there is also an even smaller % chance that rooms, enemies and the like will generate chests, there are 2 types of chests

Basic chest: Generates 2-5 pickup items, can also appear in mini puzzle rooms

Locked Golden chest: Generates 2-8 pickup items, can also generate "special items"

Special items:
Special items are the core of the games "leveling system". Similar to Zeldas leveling system (if you can call it that) Isaacs growth focuses on finding items that stack together, raise your stats, change how you play and how your character appears in game.

There are over 110 special items in the game that fit into these categories.

Usable items:
these items all have different effects when you use them and recharge by completing rooms. items have different charge rates depending on the power of the item, you can only hold one Usable item at a time.

Weapon items:
these items change your shots adding properties like homing, poison, knock back, boomarang etc. They might also totally change your attack to something very different.

Stat padding items:
these items raise your range, damage, rate of fire, speed and HP (adding container hearts)

Passive items:
these wild card items will effect many aspects of gameplay without having to do anything, they range from gaining the ability to see through walls, to the ability to fly.

these items act as "Options" (like in Gradius), following the player+shooting, blocking, finding/generating pickups or just running around killing stuff.

The majority of special items are found in treasure rooms but, less common items are also found exclusively in other rooms:
-The Shop has a set of exclusive shop items that usually aid ease of level traversing
-The secret room has a set of more odd and rare items inside it
-The Boss room has a set of stat padding items to make sure your character's stats always get raised.

There are also small sets of very rare items exclusive to other "things" but i wont spoil them.

and finally...


As mentioned before, every map in the game will feature a boss room where you face off against one of 20+ bosses, killing the boss will get you a stat padding item and open the trap door to the next level.

Like everything else in the game, bosses are also randomly chosen and have random elements within them.

Every time a map is generated, it picks a boss from a set of 4-5 bosses for that chapter set. each chapter consists of 2 levels, if it's the 2nd level of the chapter the boss that is picked gets slightly buffed HP wise to balance difficulty.

The bosses level is randomly chosen from 8 different arena layouts, sometimes appearing with "helpers" or small mobs. Like enemies, Bosses can also appear as alternate special bosses with modded stats, attacks and visual changes always keeping you on your toes.


All in all the stacking of these elements along with extra "special happenings" and other undescribed extras makes this formula of random generation create a game that is so different each time you play it that the penalty of having to start over from scratch really becomes a reward, because every time you start a new game the adventure feels totally fresh and mysterious.

There is another huge element to all of this that rewards players even more, but ill hold off on explaining that till the next post.

If you have any questions feel free to ask them in the comments.

The Binding of Isaac will be launching on Steam (Pc/Mac) in the next 2 weeks.



Kantus said...

What do Troll bombs do?

Edmund said...

they troll you...

Load*.* said...

This makes me happy. Will we be able to pre-order off Steam or will it just be available when it is? No more word on pricing yet?

Edmund said...

its going to be $5 bucks, no preorders.

Kithrus Crases said...

Theirs one thing you didn't tell us which is how does the game work as far as flow goes.

In my mind I picture a semi hub world which is randomly generated but stuck once the game starts like Terraria. After which has the levels/dungeons we can enter as we see fit regardless of order.

Or am I wrong?

Edmund said...

there is 8 levels in the full game that go straight down from the starting chapter, these chapters are linear in theme, so ch1 will always be the basement, ch2 the caves and so on. the content of each chapter is always different but the chapters aren't randomized or it would disrupt the difficulty curve.

Unknown said...

I can't wait to play the game.
LOVE SMB and have been a huge fan of the art stlye, ever since early newgrounds days :)

take my mone, now!! ;)

Steven said...

Thanks for the detailed post. I'm getting very excited--Does Steam allow you to pay more for a game??

Also, when I think of this game, for whatever reason, his royal pee-wee comes to mind: "When you've gone over something again and again ... and again and again ... like I have ... certain questions get answered. Others spring up! The mind plays tricks on you. You play tricks back! It's like you're unraveling a cable-knit sweater ... that someone keeps knitting ... and knitting and knitting ... and knitting and knitting and knitting!"

Robert D. said...

Hi Mr. McMillen!

I played SMB and some other stuff on You got an awesome style, I am so unbelievable excited about The Binding of Isaac, I can't handle my patience any longer.

So I got 2 questions:

Will there be another gameplay trailer?
When will the next blogpost be released? :D

Thank you for this eye-opening blogpost, greetings from Germany!


jw said...

Nice post, good to read some game designing that makes sense now and then.

Please tell me you are incorporating some kind of roguelike style legend making about the final boss at least? Beat the ... in level 8 etc. Or is the final boss also randomized? Loved that in RL's, especially if you never make it past 5% of the game, and it rumours of even more shit after the final boss.

Anonymous said...

In a game about shooting aborted fetuses which spawn flies that shoot poop at you. What do you think the most potentially offensive thing in the game is? =D

Graham Goring said...

This sounds so awesome. :) Day one purchase!

Does it track which things you've found out of the hundreds of them so you know if there's anything left to discover?

Anonymous said...

To be honest, I know about this game only because I follow Team Meat on Twitter and I love meatboy. At first I wasn't so sure if this game was bad, but shame on me, it does look and sound like a must buy. Will be getting it ;) . Great job.

Anonymous said...

Will it be $5 in the UK (~£3.50) or will it be £5, which is actually more like $8?

Unknown said...

you should be able to pick 1 item/stat to keep from your previous game when you start a new game :D

Anonymous said...

Thank you for the Mac version! Can't wait to play your game (many times =).

I just hope it's not too hard. Some kind of permanent upgrades in a separated (maybe easier) mode would be nice, if that makes sense from your perspective.

crusader8463 said...

Will there be a demo? I don't understand this game and would like to try it out before I throw money at it.

Anonymous said...

I would like to note that this blog post (linked via RPS) has changed the game from something that, to be honest, was of no interest to me into pretty much a must-buy. It seems amazingly interesting, if also mildly distubing...

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By any chance did you ever release said script?

Yura said...

From how you're describing it, the Binding of Isaac sounds like a mix of Zelda and Spelunky - can't wait to put down $5 for it in a few weeks!

Anonymous said...

when you respawb the maps generated randomly, right?.
What about the items?, do you lost everything if you died?

Edmund said...

@Robert D. Yes, we are working on a straight gameplay trailer now to launch along side the game.

i hope to do another blog post in about 2-3 days. and i hope ill also announce the release date then as well

Edmund said...

@jw there are 2 final bosses actually. and endings for killing them. it will be in my next post.

@Graham Goring yes, there is a collection page that shows icons for items you have collected ones that are locked and you over all % of items found.

@crusader8463 there will be a demo but due to crunch it wont be out for a week after release

Robert D. said...

Thanks for the fast reply, now I can sleep well. :)

Looking forward to all the stuff coming these days, keep up the good work!

RodrigoPG said...

I am just commeting to say that this game will be awesome! Thanks edmund and the rest of your little but competent team, and you can confirm my first-day buy! :D

Keep the good work and congratz, I hope all good for you.

asciidreamsfan said...

Okay, so a release date in 2-3 days. That makes me happy!
Now, are you going to add diggles at any point? I am actually thinking of waiting till you add them to buy it... No, I'm joking.

I love rogue-likes. I like where you are coming from for this game. I'm going to instantly buy it.

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Edmund, I was looking over other games you've made just now. I seem to have been a fan for quite a while. I've been stalking your blog and twitter hoping to get some updates on this. I'll be picking up the game as soon as it's out on steam. Looks amazing so far.
Keep up the great work

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Robert D. said...
This comment has been removed by the author.
Graham Goring said...

Awesome, cheers. That scratches my collector personality nicely. :)

Robert D. said...

Ah, 2 other questions just popped into my mind again:

Will there be updates like little or even big DLC's so new items, enemies or stuff like that will be added?

That would be superawesome, because I think after the 100th time you play this through you might have seen all items, enemies et cetera.
I would even pay again for a big DLC. :D

Will there be achievements? :)

nihilocrat said...

Looks really awesome, the kind of game I have been waiting for since Spelunky. Good luck with the crunch!

Edmund said...

@Robert D. yes, there will be free dlc updates over time for sure, new bosses, items characters etc.

yes there will be over 40 achievements at launch.

asciidreamsfan said...

Will there be Diggles?!

Edmund said...

what the hell is a diggle?

asciidreamsfan said...

Ah sorry Edmund, forgot you don't do teammeat twitter.
Just obsessing over Dungeons of Dredmor (another recently released indie rogue-like) till your crunch ends and I get to obsess over Isaac.

Robert D. said...


An awesome idea, awesome graphics, a pretty huge amount of items, enemies, bosses et cetera, achievements, some dlc updates to provide almost endless randomness for the future...
And all that for 5 bucks!

People must be insane if they won't buy this little diamond, hail to indie games!

Unknown said...

While I was pretty comfortable understanding this variety of interpretation from earlier statements, it is nice to have someone just come out and give a nice description in detail.

This game development has been looking forward to picking it up at release.

CZ3RO said...

You had me at roguelike. Even if it's just roguelike-like. :]

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Unknown said...

Very cool game,
Its will be interesting to know some stats... like for example the % of luck to get some item, or boss...
just so you can make good strategies!

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is there some "random" thing in room too? or are all allready done?

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