Reverse Engineering SimTower

This is the story of building https://towers.world, a ~perfect, tick-for-tick reproduction of 1993 game SimTower.

I spent weeks reverse-engineering the original EXE and writing up a detailed spec of how the simulation actually works under the hood — population flow, elevator AI, the whole star-rating system. The specs are all on GitHub if you want to read them: https://github.com/phulin/tower-together/tree/main/specs

It's also collaborative. Multiple players can connect to the same tower, the simulation keeps running as long as anyone's connected, and build actions sync across clients in real time.

Because I rebuilt the UI, I could add features like shift-click to build grids of rooms - instead of painstakingly clicking to build rows of offices and hotels, shift-click allows a 10-floor grid to be built all at once.

The game runs on Cloudflare Durable Objects, one of my favorite web primitives.

Everything is open source: https://github.com/phulin/tower-together

phulin.me

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209 points

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patrickhulin

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3 days ago

52 comments

johnfn ・ 10 hours ago

First of all, super cool. I have a soft spot for SimTower as well. :)

> I didn’t want to do a function-by-function port. First, APIs may be copyrightable - and copying a binary that closely might implicate copyright more than an approach closer to clean-room design. But it was clear that I needed some level of feedback from the ground-truth binary in order to provide a hill for the LLM to climb on the reimplementation.

Interesting, but isn't this what, say, the Ocarina of Time reverse engineer port does[1]? I imagine the fact that this hasn't been served a takedown notice from Nintendo is a proof that it's defensible? Or at least that there's precedent, ha.

Anyway, this is really cool. I genuinely think the only thing that's missing for me to waste an afternoon here is the sound effects!

[1]: https://github.com/zeldaret/oot

  • MrDOS ・ 24 minutes ago

    > APIs may be copyrightable

    Didn't Google v. Oracle disprove this?

  • charcircuit ・ 3 hours ago

    It is not proof. It is a clear derived work infringing on the copyright of Nintendo.

    • realusername ・ 2 hours ago

      Depends of the country, a lot of countries have exceptions for interoperability (at least the whole EU) and since these projects are mainly used to make ports to other systems, it may be covered.

jlev1 ・ 14 hours ago

Wow!

There was a little-known sequel to SimTower called Yoot Tower (named after Yoot Saito). It was a commercial flop, but I played it in the 2000s and again in the 2010s and very much enjoyed it! It had a lot of added customizability (more choices of restaurants and shops, for one thing). I would love to see that game recreated.

  • zamadatix ・ 12 hours ago

    Project Highrise is also worth checking out. It's a 2016 re-imagination of the same type of game. I'm not sure I enjoyed it as much as I did Yoot Tower... but I'm also not sure if that's just because Yoot Tower was a few decades younger back when I played :).

  • pixelcort ・ 13 hours ago

    One of the maps in Yoot Tower is Kegon Falls. When I moved to Japan I found out it was a real place. Just took another day trip up there last month. Of course IRL it’s just an elevator but the view each season is amazing.

  • thispbowden ・ 14 hours ago

    Love Yoot Tower! Can we clone it next?

    • patrickhulin ・ 13 hours ago

      yes! I've been thinking about how to do it, but it will take some work!

dsl ・ 4 hours ago

It is missing the most important function.

If the first tile you build is a lobby in the bottom left corner, it is supposed to double your starting money. :)

  • LandR ・ 21 minutes ago

    Was this a thing?

    I never knew this!!

    Man, pre big-internet was so hard to find information on games. I remember for the original tomb raider a friend needed a guide, so I wrote and printed out them a guide for the full game, since I played it pretty obsessively.

  • ashdnazg ・ 4 hours ago

    The real question is how many other things it's missing.

    Reading the process in TFA, it's very much dependent on the comprehensiveness of the testing framework. And apparently, the tests never built a lobby in the bottom left corner...

    Anything else it didn't try, is probably also not documented and not implemented.

    With the growing use of AI in reverse engineering, we might need to shift our goals to more strongly verifiable ones, such as matching decompilation.

  • disillusioned ・ 3 hours ago

    Literally the first thing I tried. :-/

swyx ・ 8 hours ago

> This project used an absolutely ridiculous number of tokens. I had to upgrade to the Claude Code $200/month plan and carefully avoid usage in the 8 AM-2 PM 2x peak window.

that doesnt sound too terrible to be honest. TIL that 8am-2pm is 2x usage.

grogenaut ・ 8 hours ago

I played when I was a kid. I figured out you could just build condos like crazy and make money pause build more, and repeat, get tons of money, build all the way up. But I couldn't figure out how to get people to the top level for that achievement.

I'm gonna Google that and see what I got wrong

  • manwe150 ・ 7 hours ago

    I see to recall it being an express elevator to floor 90 (multiple of 15), then a short regular elevator for the rest of the way to the cathedral. I beat the game several times, usually with the 100% lowest-cost office build.

neals ・ an hour ago

Played it for a few cycles. I feel like the elevators are a bit bugged, they keep idle in the lobby why people are waiting to get on (and turning red)

dwedge ・ 4 hours ago

I want someone to do this with SimCopter. I loved that game but I don't think it was very popular

  • sysworld ・ 2 hours ago

    SimCopter was the best game!! You could even fly around your own simcity build!

qingcharles ・ 8 hours ago

Can you talk about how you actually disassembled the EXE and reverse-engineered it?

MrPowerGamerBR ・ 10 hours ago

Just a fyi for anyone that doesn't know: A few years back Don Hopkins got the permission from Yoot Saito to open source Yoot Tower, SimTower's sequel: https://github.com/YootTowerManagement/YootTower

...however I don't think that the source was ever published anywhere, considering that the repository still doesn't have the source code yet. ("Please check out the YootTower repo, where I'll publish the source code once it's cleaned up, reviewed and approved by Yoot, and relicensed with the MIT licensed.")

  • DonHopkins ・ 4 hours ago

    I spoke with Yoot about it recently. There are some issues with Nintendo and licensing that may take some time to work through. My fingers are crossed, but it might take a while and require patience. We all want it to happen though!

    Meanwhile I'm working on Micropolis (based on the original SimCity Classic code, cleaned up, and compiled with Emscripten into WASM), and also reimplementing The Sims character animation system and content management and creation system in TypeScript with WebGPU and MOOLLM. ;)

    Micropolis WASM + WebGL tile renderer demo:

    https://micropolisweb.com

    MicropolisCore repo:

    https://github.com/SimHacker/MicropolisCore

    Micropolis design docs:

    https://github.com/SimHacker/MicropolisCore/tree/main/design...

    Sims character animation demo with WebGPU renderer:

    https://vitamoo.space

    Source code:

    https://github.com/DnfJeff/SimObliterator_Suite/tree/main/vi...

    Design docs:

    https://github.com/DnfJeff/SimObliterator_Suite/tree/main/vi...

    Designs for creating LLM driven Sims content editing and creation tools, uplifting and simulating The Sims in MOOLLM, and downloading The Sims content back into the original game:

    https://github.com/SimHacker/moollm/tree/main/designs/sim-ob...

    >"What would your Sims say if they could finally talk to you?"

    >Two-way bridge between The Sims 1 save files and MOOLLM. Characters, objects, and pets step between a 26-year-old game VM and an LLM-powered universe, retaining and synchronizing their parallel existences.

    >The One-Line Version

    >Drag a 25-year-old Sims save file in. Watch the character wake up. Have a conversation with them. Send them home changed.

    The Uplift: Sims ↔ MOOLLM Character Bridge

    https://github.com/SimHacker/moollm/blob/main/designs/sim-ob...

    It's all still a work in progress that I hack on when I have spare time (and spare money for tokens), so no ship date known! ;)

  • DonHopkins ・ 4 hours ago

    Also, Fabian Wesner's posted about using Claud Code to analyze the original micropolis repo's code and documentation to generate a new version.

    Here is a deep dive "podcast" he generated (presenting Claud's analysis in a conversational format) that describes the work. I listened to it, and it got the important trademark and licensing issues right, was quite comprehensive and technically accurate, up until the end where they said maybe there's something to learn about city planning from SimCity! ;)

    https://drive.google.com/file/d/1SGAkLIb4CQMPKU7TJ2sRdEjc0Dh...

    It pretty much parallels the analysis docs in his repo, which are also quite accurate:

    https://github.com/FabianWesner/micropolis/tree/master/docs

    This lightning talk I gave at HAR 2009 goes into what you can actually learn from playing SimCity/Micropolis (and reading and extending the code), thanks to Seymour Papert's and Alan Kay's philosophy of Constructionist Education, and it's much wider and more generally useful than just city planning:

    https://donhopkins.medium.com/har-2009-lightning-talk-transc...

    To save you the indignity of LinkedIn scraping your browser extensions, here is a copy of Fabian Wesner's post and my response with the link spying urls resolved:

    https://www.linkedin.com/posts/fabian-wesner_micropolis-simc...

    Fabian Wesner | CTO | Passionate about AI and Entrepreneurship | 1 month ago

    Claude Code is extremely good at documenting legacy software. That's a huge blessing, especially when the original developers are no longer around or are finally ready to retire.

    If you haven't tried this, you might be missing out. So I did it for you.

    #Micropolis is the open-source release of the original #SimCity, published by Electronic Arts in 1989. EA released the source code under GPLv3 in 2008 for the One Laptop Per Child project. The codebase is decades old, and there's likely no one left who knows it inside out.

    SimCity was one of the very first games I played, back in 1990 on my father's 286. Just a few months after the Wall fell. So this one is personal.

    I let Claude Code read the entire codebase and write a full documentation. As far as I can see, it looks correct and complete. I'm no expert on this particular codebase, but that's exactly the point: there might be no expert left on earth. I have tried this on other software where I am an expert, and the results are surprisingly accurate.

    You can find the resulting docs and the prompt here: https://github.com/FabianWesner/micropolis/tree/master/docs

    I also converted the docs into a podcast episode for easier consumption:

    https://drive.google.com/file/d/1SGAkLIb4CQMPKU7TJ2sRdEjc0Dh...

    Enjoy!

    I know I’m not the first to use Claude Code on this treasure and - of course - I already rebuilt the game with JavaScript.

    More on this tomorrow!

    Don Hopkins | Full Stack Generalist AI Engineer

    Although I may have two feet firmly planted in mid-air, I am still on Earth and know the code. I reviewed it and am I'm impressed with this documentation, which even caught my experimental Eliza chatbot easter egg in the PyGTK version! Importantly it correctly covered all the licensing and trademark issue, and mentioned that active development has moved on to the cleaned up and renovated MicropolisCore repo.

    https://github.com/SimHacker/MicropolisCore

    Here are the latest design docs:

    https://github.com/SimHacker/MicropolisCore/tree/main/design...

    The documentation you generated does a good job at mapping out and comparing the several different versions of the code. Albert Hofkamp and I cleaned up and refactored the code into C++ MicropolisCore (which I integrated with Python with SWIG, and later WebAssembly with emscripten/embind), and we wrote Doxygen comments and documentation in the code.

    Chaim Gingold deeply studied multiple versions of the SimCity source code for his PhD thesis in Play Design, and interviewed many people involved, then wrote the definitive magnum opus "Building SimCity". Chaim is the one person on Earth who understands SimCity better than Will Wright!

    https://direct.mit.edu/books/monograph/5791/Building-SimCity...

    >Building SimCity: How to Put the World in a Machine

    >A deep dive into the trailblazing simulation game SimCity, situating it in the history of games, simulation, and computing. Building SimCity explores the ...

    Good grief you can't hide easter eggs any more!

    "Additionally, the web version includes: PacBot -- an AI robot that follows roads toward heavy traffic and "eats" cars, reducing traffic density. This was an early experiment in programmable game avatars."

    PacBot appears one minute into this video:

    https://youtu.be/8snnqQSI0GE?t=56

    >Micropolis Online (SimCity) Web Demo. A demo of the open source Micropolis Online game (based on the original SimCity Classic source code from Maxis), running on a web...

LostMyLogin ・ 7 hours ago

> First, the AI makes premature conclusions about subsystems, records them, and then struggles to figure out when to abandon its earlier guesses.

This perfectly describes what feels off to me about Opus 4.7 (unsure if that’s what you are using). It seems to go down an incorrect path, I correct it, but it still references things from it. Trying to direct it back becomes a mess.

Has anyone experience this as well or am I going crazy? Doesn’t happen with 4.6 for me.

  • appplication ・ 7 hours ago

    I’ve had not great experiences with Opus 4.7. It seems the model is overall less effective at reliably delivering via my standard way of interacting, and it also forgets things it was asked several times, even just minutes after being reminded again (e.g. “don’t in-line imports”).

    I’m probably holding it wrong, but I think a given a sufficiently advanced AI it would essentially be impossible to use it incorrectly. Feels like a step backwards in this regard.

rho4 ・ 4 hours ago

You can only serve the 3 closest levels to a lobby with stairs or escalators.

ok_dad ・ 6 hours ago

I love you this was the best sim game when I was a kid and now I’ll spend my weekend on your site!

Dwedit ・ 10 hours ago

I remember that SimTower (Windows 3.1 version) would briefly draw the dialog boxes in Japanese, then immediately redraw it in English. This was on a computer without any Japanese fonts.

efromvt ・ 12 hours ago

Can you still build a garage in the bottom corner to get more starting money??

findalex ・ 15 hours ago

I loved this game so much. Can't wait to see Santa.

  • DonHopkins ・ 2 hours ago

    It would be great for the movie theater to be able to play any video synchronized across multiple players so you can watch it together, like the awesome movie theater in Cursor Camp. Then we could all watch the extremely weird original SimTower movies together, in their 8-bit error diffusion dithered glory:

    https://www.youtube.com/watch?v=qq8o7gg87k4

    Condom Duck:

    https://youtu.be/qq8o7gg87k4?t=211

    Psycho Party:

    https://youtu.be/qq8o7gg87k4?t=381

    Cursor Camp (recent hn discussion and neil.fun link):

    https://news.ycombinator.com/item?id=47949939

    https://neal.fun/cursor-camp/

    Also, LGR did these great reviews of SimTower and Yoot Tower, which he truly loves, so he disclaims that they are not exactly non-biased:

    LGR - SimTower - PC Game Review (15 years ago):

    https://www.youtube.com/watch?v=_4ToEDrhxo0

    LGR - Yoot Tower: The Sequel to SimTower (5 years ago):

    https://www.youtube.com/watch?v=CqNECXCd9iU

    Yoot interviewed many interesting people for MACWORLD Japan, like Joanna Hoffman, Bill Atkinson, Steve Wozniak, Douglas Engelbart, and Alan Kay. Here's a transcript of Yoot Saito's interview of Alan Kay that he shared with me and I cleaned up and linked out:

    https://github.com/YootTowerManagement/YootTower/blob/main/Y...

    A Journey Through Computing History with Yoot Saito and Alan Kay

      Yoot towers wisely,
  Alan constructs the future --
  foundations of change.
    Introduction

    In this captivating interview from 1993, recently unearthed and previously published only in Japan, renowned game designer Yutaka "Yoot" Saito of MACWORLD Japan engages with computing pioneer Alan Kay in a deep exploration of technology's past and future. This dialogue, captured on a cassette tape and transcribed, spans the evolution of personal computing, highlighting groundbreaking advancements and visionary ideas that have shaped modern technology. Yoot Saito, celebrated for his innovative approach to game design, and Alan Kay, known for his seminal contributions such as the development of the graphical user interface and the concept of the Dynabook, offer profound insights into both the historical trajectory and the potential futures of the digital world. This interview serves as a treasure trove of historical anecdotes, philosophical reflections, and forward-looking innovations, capturing a moment when two brilliant minds discussed the dynamics of technological progress.

    ----

    I'd love to find all of Yoot's original interviews published in MacWorld Japan. Yoot was very active introducing and working with technology and researchers from the US with Japan in the early days of the Mac, and he pioneered psychological chat based AI games using voice recognition and synthesis, like Seaman.

    Seaman - Sega's Strangest Expedition Into Artificial Intelligence:

    https://www.youtube.com/watch?v=VV0zpPSXOx4

    Yoot Saito on His Classic Sega Game Where You Take Abuse from a Fish:

    https://www.vice.com/en/article/yoot-saito-on-his-classic-se...

    I found a couple of MacWorld Japan CDROM archives on Internet Archive, but my Mac says it can't mount the iso files. It appears to be an old HFS file system -- does anybody know how to to mount these treasures on a modern mac, with an emulator maybe? Or have the actual magazines lying around?

    https://archive.org/details/macworld-japan-vol-1-1995-01

    https://archive.org/details/macworld-japan-vol-2-1995-09

    MACWORLD-VOL-1-1995-01.iso: Apple Driver Map, blocksize 512, blockcount 460003, devtype 0, devid 0, driver count 0, contains[@0x200]: Apple Partition Map, map block count 2, start block 1, block count 2, name Quick TOPiX by OMI, type Apple_partition_map, valid, allocated, readable, contains[@0x400]: Apple Partition Map, map block count 2, start block 3, block count 460000, name MACWORLD-ROM Vol. 1, type Apple_HFS, valid, allocated, readable

ChickeNES ・ 7 hours ago

Heh, I have four different games I am currently experimenting with, luckily I skipped this one as I am still expecting the original/Yoot source will be published (hopefully DonHopkins sees this and it nudges him to get it uploaded!).

taejavu ・ 15 hours ago

Very cool! I used to play this game for a few minutes every morning at a friends place before school. With the clone being open source, would you be open to QoL patches? I don’t have anything specific in mind, just curious how close to the original you want to stay.

  • patrickhulin ・ 13 hours ago

    Patches are welcome, especially for UI stuff - the simulation I'd like to keep matching exactly. I have one deviation already: the floors are 0-indexed and lobbies are on 15, 30, ..., which would be 16, 31, ... in the original game.

schnebbau ・ 15 hours ago

Amazing, loved this game when I was younger. I would keep restarting every time I thought of a more effective layout.

I don't think I ever made it to the cathedral though.

ryeguy_24 ・ 15 hours ago

Oh my gosh, this really brought me back. Haven't played that game in 30 years. What a great use of AI. Thanks for the trip down memory lane.

EdwardDiego ・ 9 hours ago

I sunk ages into "the world's most brutal elevator simulator" - how does the game model elevator demand? I always wondered if it was some sort of Poisson distribution.

SuperNinKenDo ・ 12 hours ago

God I loved this game back in the day. Thank you!

EDIT: Nevermind... [sigh]

  • I_am_uncreative ・ 12 hours ago

    What's with the edit?

    • cozzyd ・ 11 hours ago

      Judging from user's posting history, they're not the biggest fans of llms

      This is kind of the perfect use case though -- it's a game who cares if it's right or not.

      • DonaldPShimoda ・ 11 hours ago

        Some people also have moral issues with LLMs. If the poster is such a person, the accuracy metric may simply be irrelevant.

yesitcan ・ 13 hours ago

[flagged]

  • iamjs ・ 13 hours ago

    This is an unnecessarily rude, dismissive, and low value comment. I loved this game growing up and I think it's fantastic to see this work. I don't find it particularly relevant whether or not a language model was involved.