Super Mario Bros.‘s first couple of stages ease players into the rules of the game’s world: Walking, jumping, stomping, fireballs, secrets. With World 1-3, however, the game properly becomes a platformer. At this point, you’re expected to have a handle on the basics so you can contend with the more complex demands ahead.

World 1-3 takes place across a vast chasm dotted by a series of… trees? Mushrooms? Cliffs? I don’t honestly know. It doesn’t really matter. The important thing is that the only solid ground in World 1-3 appears at the very beginning and very end of the stage. In between, it’s all just a series of slippery platforms. Fall off an edge and Mario plummets to his death.

Looking back after 25-odd years of games inspired by Super Mario, I think it’s worth noting that the green platforms here are solid from both above and below. You can’t pass through them by leaping; instead, you have to jump cleanly around them. The brown “walls” are strictly window dressing; they serve no purpose in the game except to convey the idea that you’re not traveling across platforms floating in empty space. They actually do provide a sort of comforting presence, though, inane as that may sound — it’s strictly psychological, but I contend that the brown walls/trunks/stems/whatever create a reassuring sensation that you’re not in quite as perilous as a situation as you might be. They’re wallpaper training wheels.

But World 1-3 really is hazardous. Platforms appear at variable heights and at different distances from one another, forcing players to make bold leaps in order to progress. Coins exist not only as a power-up — and if you’ve been gathering them dutifully, you’ll finally discover here that 100 coins nets you an extra life — but also as a ruse to entice you to perform dangerous actions. Is it really worth the risk of dropping down in the narrow space between two platforms to a tiny ledge with a single coin on it? Probably not, but that jingling sound they make when you collect them just can’t be denied. It’s pavlovian, baby.

The moving platforms that served as one-way elevators reappear here, but in a different capacity. Instead of simply descending or ascending all the way off the screen, looping infinitely, they move back and forth either horizontally or vertically within a fixed space. They can take you up to higher areas or across to otherwise unreachable spaces. And unlike the green platforms, these aren’t solid ground; you can leap from beneath them to land on their upper edge. They’re more forgiving due to their more dangerous nature.

The most important thing to understand in World 1-3 is the importance of running. You can clear the first two worlds without ever pressing the B button, but forget about advancing past World 1-3 if you don’t run. In several places, you must target platforms too high and too far across a gap to jump by simply walking forward; you need the added momentum and height provided by a short run-up.

And if you’re feeling really fancy, you can experiment with the advanced technique of stomping aerial enemies. Hopping on a bad guy causes Mario to bounce, of course, so it stands to reason that would work even in mid-air. And indeed, hopping on a Koopa Paratroopa can propel Mario to a higher ledge, though of course you need to watch your timing. (There are some weird physics quirks in Super Mario that allow bizarre exploitation, like the fact that colliding with an enemy while in a downward motion registers a stomp even if the enemy hits you from above, but those are outside the scope of a piece on deliberate design lessons).

Make no mistake, World 1-3 still has its training wheels on. The most complex maneuver you need to pull off in order to clear the stage is jumping from one moving platform to the other. But these move gently up and down next to one another, alternating with the other and meeting at the midpoint of their arc. It’s child’s play to jump from the left one at the apex of its track to the right one, which will be at its nadir at the same time. But it’s an essential trick to master for later in the game, when things become far more stressful.

The color-coded enemies demand attention, too. The only Koopas and Paratroopas in this stage are red ones, which stick to a fixed range. Red Koopas never walk off edges, patrolling only their designated platform, while Red Paratroopas hover up and down along a vertical line. You can pop them with fireballs to make short work of them, or you can experiment with kicking them from uneven terrain.

At any rate, the level itself proves to be exceptionally brief. Not only is it shorter than Worlds 1-1 and 1-2, it also lacks their nuances. There are no hidden secrets here, no pipes or invisible blocks. In fact, there’s only one block in total, which disgorges a mushroom or Fire Flower as needed. The focus here is entirely on aerial maneuvers and high-altitude platforming, with no distractions beyond the shiny, shiny coins.

At the end of the stage — which is preceded by a very different sort of staircase than those of 1-1 and 1-2 — Mario enters a proper castle, not just the tiny fortresses of the previous worlds. This will prove to be a meaningful distinction.