How Rusted Moss devs teamed up to create physics-based grappling hook action

Rusted Moss is an action-exploration game developed by solo individuals (not part of any development studios) that revolves around its unusual method of traversal: an absurdly bouncy grappling hook. We won’t lie: the physics-based grappling will bring you both great pain and great joy. But the triumph once you master it — you’ll practically fly through the game while blasting enemies with your arsenal of guns when Rusted Moss comes to PS5 on June 20.

In this melancholic world, humans prepare for an invasion by capricious fairies from another realm. You play as Fern, a changeling determined to put an end to the war. As you uncover the story behind the world, you’ll eventually choose a side: fae or human?

With the PS5 release, we are also adding in seven to eight hours of additional content — approximately as much content as the base game itself. This includes new zones (ranging from moderate to very difficult), a boss rush, and an additional playable character.

A unique grappling hook

During a show-and-tell with indie developers, Emlise (the main dev) showed a grappling hook that worked like a bungee rope or rubber band. She performed incredible, crazy maneuvers that made traditional platformer abilities like double jumps and dashes seem so limited. I had never seen anything like that — grappling hooks in most games either just pull you to the anchor point or swing the player in static arcs.

It looked so polished that I was sure she would develop it into a full game. But she had no plans to. She saw it as a programming exercise to learn about verlet integration (a numerical equation used to calculate trajectories).

“Players would find it too difficult. It takes some time to get used to,” says Emlise.

Each of us then carried out our duty as friends and peer-pressured her into continuing development. My sister and I also joined her, forming our 3-person development team. It was strange because we had no intention of making a game together before that moment.

Rusted Moss was made to bring this mechanic to life, not for the sake of making a game or to go full-time on indie dev (my sister and I work day jobs outside the game industry).

I think its origin gave a purity to Rusted Moss’ foundation because there was no doubt as to what kind of game it could become. Everything would revolve around just one core mechanic — the grappling hook.

Please break our game

Synergy with the grappling hook became the focus of Rusted Moss’ game design. The abilities you gain all enhance the physics-based traversal — whether it’s a charge jump that lets you fall further, or the kickback from your guns.

This maximizes the opportunity for player expression and creativity. Progression is not based on a simple lock-and-key solution, which is a design pattern often found in other action exploration games.

With these synergistic abilities, we’ve seen the same platforming challenge solved in five different ways. If a player is creative, determined, and skilled enough, they can “break” our game and show us moves we didn’t even consider during Rusted Moss’ development.

We took this philosophy to the extreme during our quality assurance testing period. One tester found a bug that gave them an unintentional movement ability. Rather than fix it, we added a visual effect when that ability is active. We love seeing our players discover this “secret ability” in-game.

Unusual children

The main character, Fern, has a sharp and acidic personality. Women are often pressured to be nice and think about what “nice” means to the people interacting with them. Based on our experiences as an all-female dev team, Fern is a power fantasy that opposes that narrative, sometimes to an extreme. She says what she thinks, which is often quite nasty and honestly… hilarious!

This aligns with her identity as a changeling — a fairy that has replaced a stolen baby. In folk tales, children who acted strangely were identified as changelings and treated cruelly. Nowadays we might think of them as neurodivergent or unusual in some other way. We wanted to explore themes around children turning out to be something the parents did not want or expect.