Search for “SimCity 4” on YouTube, and you’ll see players constructing multilevel highway interchanges and intricate railroad networks, or maybe even a swooping flyby of a sprawling metropolis. None of these features came with the original game, which was released just after the turn of the millennium. They’ve been developed slowly and painstakingly over the 22 years since its release by dedicated aficionados — a thriving community that continues to push the game’s limits.
The work is a labor of love, and it’s a lot of labor and love. Modder Tarkus says that after all these years, he’s drawn to the deep and complex nature of the game’s simulation, which he says remains unmatched by newer games like Cities: Skylines (2015) and its sequel Cities: Skylines 2 (2023) — and even by SimCity 4’s own sequel, which was released in 2013 and simply called SimCity. Modder Ulisse Wolf says that, for him, it’s ultimately a question of community. Perhaps because of the game’s age, he finds that SimCity 4 enthusiasts are “more mature … guaranteeing you a quiet place to live in [peace] with other members.”
SimCity 4 was released in 2003, the fourth in game company Maxis’ series of city-building games. It was reasonably successful, garnering positive reviews and selling well enough for Maxis to release an expansion pack later that year. The ability for players to add their own content to the game was a key part of SimCity 4’s appeal from the start: it came with a set of tools that allowed players to create and add their own buildings to their cities. Players soon began sharing their creations online via exchanges hosted on fan sites like Simtropolis and SC4 Devotion (now defunct, but relaunched recently as SC4 Evermore), as well as discussing tips and ideas in the accompanying forums.
“It became clear to me that if I wanted to see more happen on the transportation modding front, I needed to learn how to do things for myself.”
Players soon began to look beyond the officially supported methods of adding their own content to the game and wondering what else could be done. Could the game’s road system be expanded? What about the railways? Early modding efforts were made easier by the way the game stores its data, which, as modder Matias93 explains, is in a database whose format was shared with previous Maxis games. The database stores information about the game’s various elements: for a type of road, the database would have entries for everything from which textures to display on screen when the player placed the road down to how much traffic the simulation would allow it to carry.
The game’s built-in tools provided no way to access this database, so the community developed its own — no easy task, as Matias93 explains: “[This] required the development of a veritable suite of tools able to decrypt hexadecimal symbols into legible databases, and to import and export multimedia files from and to them.”
As early modders began to find their way around the database, however, they started to find entries for systems that weren’t in the game: features that were either unfinished or never implemented. “SimCity 4 shows clear signs of having gotten less content and expansions than its creators [intended]. While some of these elements are almost irremediably broken … many others were relatively at reach to be repurposed,” Matias93 says.
Using the tools they’d developed to tweak existing entries, modders were able to build the rudiments of these unfinished features into entirely new ones. One of the first such features to be repurposed was an unused road type, apparently intended to be a dirt road for use in rural environments. SimCity 4 comes with several types of roads for players to place in their cities: suburban streets, two-lane roads, two-lane one-way roads, four-lane avenues, and highways. Players soon chafed against the limitations of the latter: there was no way to build a highway with a variety of lane widths, and connecting a highway to a road network required the use of one of several prebuilt interchanges. Players who wanted to design and build their own interchanges, or construct highways with a variety of widths, were out of luck.
The discovery of the unused dirt road meant that there was space for one more road type, and modders used it as the basis for what became the “real highway” mod, a more flexible and customizable alternative to the game’s built-in highway system. Suddenly, players had multiple highway widths to choose from, as well as modular pieces they could use to build their own custom interchanges.
Tarkus recalls coming across an early version of the mod in 2006: “I was absolutely floored. [Even though] it was an extremely limited mod at the time, I could tell it had potential.” Real-life commitments took him away from the game for most of that year, and on his return, he found that development had stalled. “It became clear to me that if I wanted to see more happen on the transportation modding front, I needed to learn how to do things for myself.”
Today, Tarkus is the lead developer on the Network Addon Mod (NAM), a sprawling package that includes the RHW mod along with a huge amount of other transport-based content: pedestrian malls, a new railway system, roundabouts, canals, bridges, and underpasses, as well as a comprehensive reworking of the game’s traffic simulator. The mod remains in active development — its 49th version was released in March 2025 — and is maintained and developed by a constantly shifting and evolving team, whose membership spans the globe.
Lucario Boricua, who spearheads development on several of the mod’s features, lives in Puerto Rico and is a civil engineer by day. He estimates that, as of early 2025, the team has around 25 full-fledged members, along with other part-time contributors, playtesters, etc. The community is friendly in general, too — members gather in both the long-running forums and newer channels like Discord, and both are generally nice places to spend time, with veteran members welcoming new fans and what Wolf calls “political bullshit” notably absent.
It’s not just the NAM that remains in active development, either. In 2025, SimCity 4 modding continues to evolve and encompass new possibilities, largely because of improved access to the game’s source code. Matias93 explains that “the first 15 years of modding, more or less, [had nothing] to do with coding per se.” Instead, they involved customizing and extending the game’s databases. However, that changed in the 2010s because of two key developments.
The first was the release of a portion of the game’s code, provided by the late Paul Pedriana — the game’s original chief programmer — in response to a request from the community. This allowed for the reimplementation of some of the game’s internal debugging features, which were essentially cheat codes that allowed developers to do things like quickly set up cities without worrying about practicalities like having enough in-game cash, place specific buildings instantly, and many other useful features. Then, in 2019, a 64-bit macOS port of the game was released — with an unexpected bonus for modders. “A lot of the game code [was] legible on the released executable, allowing modders to identify certain possible hooks to work with,” Matias93 says.
Modders continue to lack full access to the game’s code, but even limited access has allowed the use of techniques that would have otherwise been impossible. Foremost among these is the use of dynamic link libraries (DLLs), which allow custom code to be loaded into the game’s executable at runtime. The ability to write custom DLLs has allowed modders to fix long-standing bugs in the game — most notably, one notorious issue that could lead to save game corruption in large cities. It also let them dream up features that would not have been possible otherwise: recent additions include a 3D camera, the reimplementation of the game’s long-broken water system, and a Steam Workshop-esque interface for finding and downloading custom content.
Nevertheless, the process of creating these features remains far less straightforward than for modern games, which often come with dedicated development kits designed for modders’ use. So what is it that keeps the NAM team and its fellow modders working on SimCity 4, painstakingly implementing new features and testing the limits of what’s possible?
Lucario Boricua says that working on the game today feels like “standing on the shoulders of giants.” The decades of work that modders have put into the game provide a platform for today’s creators to push the game into places its designers probably never imagined: “We’re focused on doing revolutionary things that redefine what’s possible.”
