Black Hole Square

Black Hole Square opens with no menu, no settings screen and no explanation beyond one line of text: "Swipe left to start." That single sentence turns out to be almost the entire manual. Built by Quinten Clause for js13k 2021 — the competition where every entry has to fit in a 13-kilobyte zip file, theme "SPACE" that year — it's a grid puzzle where the whole interaction model is a tap to act on a piece and a swipe to move between puzzles. There's no score, no timer pressure, no account, nothing to buy. It is exactly as small as it sounds, and it spends that smallness on getting the one thing this shelf cares about most right: it actually works with a thumb.

Eighteen puzzles, and what they're really made of

The README talks about "cleaning up all the other squares in the universe," which undersells what the game actually asks of you. Reading the shipped level data directly — the grid layouts and par move-counts baked into the production build, not just skimming the source repo — shows 18 distinct hand-built puzzles on a 6x6 board, each combining pieces from six behaviours: tap an X-square to delete it, tap a neutron star to collapse it into a black hole that then swallows anything pushed into it, and four directional arrow tiles that shove every pushable piece in their row or column one space in that direction when tapped. Nothing here is randomly generated — every grid is a specific, deliberate arrangement, and the par-moves counter at the top of the screen (4 moves for the first puzzle, 20 for the last) is baked into that same level data rather than computed live.

The difficulty curve that emerges from playing through it is real but not smooth. The first six levels climb gently — 4, 5, 6, 5, 6, 6 moves — teaching the delete mechanic and each of the four push directions in turn. Then "The wander," puzzle seven, suddenly asks for a 15-move solution, a jump that has nothing gentle about it after the levels before it. The very next puzzle drops straight back to 5 moves. From there the back half of the game climbs again in a genuinely authored way — 10, 5, 9, 11, 9, 6, 8 — before the final three puzzles commit to being the hardest in the game outright, at 12, 14 and 20 moves. That's the signature of a human designer working level by level inside a size budget, not a generator smoothing out a curve — which is a point worth making plainly, because it's exactly the kind of claim a review of a 13-kilobyte game can actually verify instead of guessing at from feel.

One correction worth being explicit about: the project's own README still advertises "unlock 3 bonus puzzles by supporting this game through web monetization," and the source repository's entities.json file appears, at a glance, to gate those levels behind a payment flag that's never set anywhere in the code that could set it. Reading only that file would produce an inaccurate review — the game would look like it locks a chunk of its own content behind a now-defunct payment API. The actual production build tells a different story: its level sequence has already been edited to include every one of those puzzles for free, and playing through it on both a simulated phone and desktop reached all of them without a monetisation flag ever being set. Whatever the original 2021 competition build did, what's hosted and playable today asks nothing of anyone.

Controls: the part that actually matters here

This is where the game earns its keep. Every tap in testing landed on the exact grid cell it was aimed at and produced the exact effect the source code says it should — an arrow tile shifted the correct row or column by exactly one space, over and over, with the move counter ticking down in lockstep. That was tested with real touch events on a simulated phone, not a mouse pretending to be a finger, and then tested again with an actual mouse click on a desktop viewport, landing on the same tiles and producing identical results. Swiping is handled the same way: a deliberate drag past a distance threshold resets the current puzzle or advances to the next one, and it never mistook a tap for the start of a swipe or vice versa across either device profile. There's no long-press mechanic to test here — everything the game does is a single tap or a single directed swipe — which is itself a small mercy, because it means there's no touch gesture in the whole game that a phone might interpret differently than the developer intended.

The interface backs this up with small, correct decisions: a strip of dots beneath the puzzle board tracks progress through all 18 levels at a glance, the move counter gives instant feedback on whether a solution was efficient, and swiping past the current puzzle in either direction always tells you in advance ("Skip" or "Reset"/"Back") what that swipe is about to do before you commit to it. None of this is flashy, but all of it is legible on a phone screen at a glance, which is a harder bar to clear than it sounds for a game built by one person in a code-size jam.

What eighteen puzzles actually buys you

The honest limitation is length. Eighteen levels, most solvable in under a minute once you've worked out the trick, adds up to something closer to a fifteen-to-thirty-minute sitting than a game with any real staying power. There's no random puzzle mode, no daily challenge, no way to generate a nineteenth level, and swiping back through completed puzzles only lets you replay ones you've already solved rather than offering anything new. That's not a flaw in execution — it's an honest reflection of what a 13-kilobyte competition entry can be — but it caps how much value there is on offer regardless of how clean the controls are. Visually the game commits to one flat, dark colour scheme and one icon set for its entire runtime; it's legible and unfussy rather than memorable, and nothing about puzzle 15's presentation differs from puzzle 3's beyond which pieces happen to be on the board.

The registry note that flagged this game for review described it as a js13k 2021 Mobile-category winner, and that would be a real, relevant signal if it holds up — winning a category judged by hundreds of peer developers is a much stronger endorsement than anything a review can offer on its own. It could not be confirmed here: the competition's own results now live behind a JavaScript-rendered site that returns nothing to a direct, non-browser fetch, contemporary coverage of the 2021 winners doesn't name a Mobile-category winner by title, and the developer's own portfolio page for the game makes no mention of any award. What can be verified independently is more modest but real: the repository sits at 66 stars and 27 forks with zero open issues as of this writing, and the game was one of twenty entries a widely-read developer publication chose to spotlight from that year's competition. Treat the "winner" claim as unconfirmed rather than repeat it as fact.

Verdict

Black Hole Square does not try to be more than it is, and what it is, is a genuinely well-built small puzzle game with touch and mouse controls that work exactly as advertised, wrapped around 18 hand-designed levels with a real if occasionally lumpy difficulty curve. That's enough to be worth the short time it takes to play through, and the controls in particular are a legitimate example of what "actually playable with thumbs" looks like when someone builds for it from the start rather than bolting touch support on afterward. It's not enough to be more than a short, free diversion — there's no reason to come back once the eighteenth puzzle is solved, and the presentation never varies enough to make revisiting it tempting on its own. Worth the fifteen minutes; not a game to expect a second session from.