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Play Ossa: The Giants' Stair — a free Greek-myth stacking runner

Climb the stair the giants stacked to heaven, spend your stones to clear rubble and chasms, duck the low sky-gates and cut Artemis free. Thirty chapters, four acts, an endless ninth heaven.

Ossa: The Giants' Stair is a free browser-based stacking runner built on Greek myth. In the myths, the twin giants Otus and Ephialtes — the Aloadae, sons of Poseidon — piled Mount Ossa on Olympus and Mount Pelion on Ossa to make a stair to heaven, then carried off Artemis, goddess of the moon and the hunt. Our story starts the night the moon went out. You are Aethon, a mason's son from the Vale of Tempe. The temple where you cut stone for the goddess is dark, her hunters are scattered, and the giants' stair is still standing. So you climb it, one carried stone at a time.

The loop is one button and two numbers. Leap to cross rubble and chasms; height decides what you can cross and what you cannot. A rubble wall of three stones costs three stones to clear, a chasm two wide costs two, and every stone you spend is gone — you rebuild the stair by catching marble shards that float along the ridge. Then the giants' low sky-gates arrive, and they kill the obvious strategy: a stair taller than the gate's clearance is stopped dead. The game is not about getting tall. It is about arriving at each obstacle at exactly the height it asks for.

How to play

1. Leap with one button. Press Space, ↑ or W, click the stage, or tap anywhere on a phone. You are airborne for 0.46 seconds; rubble, gate and chasm checks happen the moment your body reaches them.
2. Read the ring. A gold ring closes on the next obstacle. Leap when the ring is on the stone: too early and you land in front of the wall, too late and you are still rising when your chest meets it.
3. Pay in stones. Rubble costs its height in stones, a chasm costs its width. Land a perfect leap — a ring inside the last fifth of the window — and you get one stone back plus a gold chime.
4. Duck the gates. A sky-gate prints its clearance in stones. If you are taller than the clearance you lose every stone above the lintel and one of your three chances, so shed height before you get there: a gate is a wall you survive.
5. Charge the moon. Moon fragments fill the silver bar. Spend it on Artemis' Aim: the world drops to 55 percent speed for two and a half seconds, which is enough to read a double wall or a chasm in the dark.
6. Outrun the storm. Three stumbles and the storm at your back takes you. Reach the summit gate with none and the chapter is worth three stars.

Controls

Input	Action
Space ↑ W · click · tap	Leap. Also vaults rubble and bridges chasms if your stair is tall enough
Shift or E	Artemis' Aim — slow the world while the silver bar drains
Esc or P	Pause
R	Restart the chapter
M	Mute the synthesised score
On-screen Aim button	The same slow-time power on a phone or tablet

The rules, exactly

Thing	What it does	What it costs
Marble shard	Adds one stone under your feet, up to eight	Nothing — but high shards need height to reach
Rubble wall	Must be leapt, and your stair must be at least as tall as the wall	Its height in stones
Chasm	Must be leapt; the stair is dropped in as a bridge	Its width in stones
Sky-gate	Passes only if your stair is shorter than the printed clearance	Every stone above the lintel, plus one stumble
Moon fragment	Fills one third of the Aim bar and scores 250	Nothing
Perfect leap	Ring inside the last fifth of the window	Refunds one stone, scores 150
Stumble	Crash, fall or gate collision; 0.8 seconds of recovery	One of three. The third ends the climb
The storm	Follows you the whole way up; every stumble brings it closer	Ends the run at three

How it differs from other endless runners

	Ossa	One-button jumpers	Stacking towers
What you manage	Height as a currency: earn it, spend it, shed it	Timing only	Accuracy only
Failure	Wrong height for the obstacle, not a missed tap	A missed tap	A misaligned drop
Level design	30 authored chapters with a measured stone budget	Endless procedural	Endless procedural
Ending	Chapter XXX: the chain is cut	None	None
Story	Four acts of Greek myth, told on the stair	Backdrop	Backdrop

The four acts of the stair

Act	Chapters	Where you are	What changes
I · The Vale of Tempe	1–8	Dawn in Thessaly, the foot of the stair	Rubble only; the ring teaches itself
II · Ossa's Shoulder	9–16	The first mountain's spine, gold light, storm gathering	Sky-gates appear; height becomes a liability
III · Pelion's Spine	17–24	Broken ridge, night, wind through the gaps	Chasms, gusts, tighter spacing
IV · The Iron Sky	25–30	Above the weather, under a chained moon	Darkness, double walls, gates in sequence

Why every chapter can be finished

Every chapter is generated against a stone budget and then run by a bot. The generator only places a rubble wall or a chasm if the stones it costs are already in your hands, and it plants shards to refill the stair before the next wall. After that, a reference bot plays the chapter with the same physics the player gets — same jump arc, same collision rules, same stumbles — and has to reach the summit gate with zero stumbles. The verification suite also records the tightest leap window in every chapter in milliseconds, and nothing ships under 180 milliseconds, which is roughly the reaction time of a person who is paying attention. The three-star line is therefore a measured line, not a marketing one.

Strategy that actually works

• Travel light into gates. If a gate reads 4, you want two or three stones, not seven. Clear the wall before it and spend the change.
• Bank stones before a gap. A chasm is a bill, not a jump. Arriving with one stone at a three-wide gap is a fall, however clean your leap.
• Hunt high shards when the next gate is high. Shards that float at head height are only reachable with two or more stones — they are the game's way of paying you for being tall at the right moment.
• Save Aim for the dark. In Act IV you cannot read the next obstacle until the ring appears. Two and a half seconds of half speed is worth more than a whole chapter of reflexes.
• Perfect leaps are free stones. The refund is one stone, every time. On a chapter with four walls, four perfects is a whole extra wall you can afford.

Technical notes

Everything is generated: no image is loaded, no audio file is played, no third-party script runs. The mountains, the storm, the marble and the stair are Canvas 2D drawn from the same level data the reference bot uses, and the wind, the stone clack, the chime and the low drone are synthesised with the Web Audio API. The rules contain no random numbers — every shard, wall and gate comes from a seeded generator, so a chapter plays identically on every device. Progress and stars live in your own browser under ossa.stair.v1. Nothing is uploaded, because there is no server.

Ossa: The Giants' Stair — frequently asked questions

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