Marble Sort Level 21 Walkthrough & Solution

Level 21 Board Overview: The Twin Tunnels and the Mystery Wall

Level 21 greets you with a wall of question marks. A chunky 3x3 block of gray Mystery Boxes sits right in the middle of the Top Tray, and a Tunnel with a countdown of 3 guards each flank. Until those Tunnels fire, most of the board is a rumor.

The colors in play are Blue, Black, and Green — plus one very smug Red block parked in the middle of the front row. Your exposed front line, left to right, reads: Black, Green, Blue, Red, Black, Blue.

Down below, the four receiver columns open with two Blue Boxes on the left, a Black Box in the middle-right, and a Green Box on the far right. So of the six blocks you can touch, four have a home, one color repeats, and one — Red — has absolutely nothing waiting for it.

The Trap in Marble Sort Level 21

This board wants you to treat the front row as a free-for-all. It is not.

• The Red block: dead center, easy to tap, and completely homeless. There is no Red Box anywhere near the top of the receiver columns. Drop it and it squats on your Conveyor Belt for half the level.
• The left Tunnel: needs 3 clears. Behind it waits a column of three Light Blue blocks and one Pink — your entire late-game supply for those colors.
• The right Tunnel: also 3 clears, hiding Green, Brown, Brown, Green. A staggered column of Brown, Green, and Blue hangs off the far-right edge.
• The receiver columns: each is stacked five deep (Blue over Yellow over Pink over Light Blue over Green on column 1, and similar mixes on the rest). What you see now is not what you will need in thirty seconds.

The math is tight: the front row gives you exactly four safe clears — two Blue, one Black, one Green — and the Tunnels need three each. That is your whole opening, with zero room for a wasted tap.

Marble Sort Level 21 Step-by-Step Walkthrough

The opening is a fixed sequence. Improvise later, not now.

Opening: feed the Tunnels exactly what they want

1. Tap the Blue block in the front row. It drops into one of the two Blue Boxes on the left.
2. Tap the second Blue block. Both Blue Boxes clear, and both Tunnel counters tick down.
3. Tap the far-left Black block into the Black Box.
4. Tap the Green block sitting above the Green Box on the far right.
5. Both Tunnels hit zero and swing open. Do not touch the Red block — it stays put.

Mid-game: peel the Mystery grid from the bottom up

With the front row gone, the 3x3 Mystery grid becomes your new front line, and the receiver row has rotated to Yellow, Yellow, Green, and Black.

6. Drop the bottom row of Mystery Boxes and see what they reveal. Anything showing Yellow, Green, or Black goes straight into its Box.
7. The right Tunnel is now feeding out Green and Brown marbles. Column 4 still wants Green — route those Greens there.
8. Pull Mystery Boxes one row at a time and match only what the current front row of receivers asks for. Unknown colors on a crowded belt are how this run dies.

Cleanup: empty the flanks, then deal with Red

Once the grid is gone, the receiver columns start calling for Brown, Pink, and Light Blue.

9. Work the left flank: the Light Blue and Pink blocks stacked behind the old left Tunnel clear columns 1 and 3.
10. Work the right flank: the Brown blocks from the right side and the staggered far-right column finish the mid-layers of columns 2 and 3.
11. When a Red Box finally cycles to the front, tap that Red block you have been ignoring since the very first move. Belt empty, board done.

Level 21 Tips Recap

• The front row holds exactly enough safe clears to open both Tunnels — two Blue, one Black, one Green. Spend nothing else.
• Red is bait until a Red Box physically exists. Patience beats panic.
• Reveal Mystery Boxes one row at a time so the belt never carries more than one orphan color.