Marble Sort Level 21 Walkthrough Solution | Marble Sort 21

Marble Sort Level 21 Guide: The 3x3 Mystery Grid and Dual Countdown Tunnels

Marble Sort Level 21 hits you with a massive wall of hidden information right out of the gate. The dominant colors in play are Blue, Black, Green, and a highly suspicious Red.

The Top Tray is dominated by a massive 3x3 grid of grey Mystery Boxes sitting dead center. You cannot see what colors they hold. Flanking this central grid are two Tunnels, both stamped with a "3". This means you must clear three bottom Boxes before those Tunnels will open and release the marbles trapped inside. The frontline of the Top Tray is fully exposed, but dropping the wrong color here is fatal. The entire level revolves around clearing the safe front marbles to trigger the Tunnels without accidentally dumping dead colors onto your Conveyor Belt.

The Dual Tunnel Countdown in Marble Sort Level 21

You need to know exactly how this board is stacked to avoid an early game over. The layout is a deliberate trap designed to crowd your Conveyor Belt.

Look at the Top Tray. The front accessible row reads left to right: a Black block, a Green block, a Blue block, a Red block, another Black block, and another Blue block. Behind the center blocks sits that intimidating 3x3 grid of Mystery Boxes.

The left flank is controlled by a Tunnel requiring 3 clears. Behind that tunnel sits a column of three Light Blue blocks and one Pink block. The right flank has a matching Tunnel requiring 3 clears. Behind it sits a Green block, two Brown blocks, and another Green block. Hanging off the far right edge is a staggered column containing a Brown, Green, and Blue block.

Now look at your lower Boxes. The placement here dictates everything. The four columns from left to right start with:

• Column 1: Blue, followed by Yellow, Pink, Light Blue, Green.
• Column 2: Blue, followed by Yellow, Brown, Black, Yellow.
• Column 3: Black, followed by Green, Brown, Pink, Yellow.
• Column 4: Green, followed by Black, Green, Green, Pink.

Your active frontline on the bottom consists of two Blue Boxes on the left, one Black Box in the middle-right, and one Green Box on the far right.

Notice the massive disconnect. The Top Tray offers a Red block right in the middle of your accessible frontline. Look at your lower Boxes. There is no Red Box anywhere near the top of those columns. Red is pure bait. If you touch that Red block, it will drop onto the Conveyor Belt and sit there for half the game, eating up precious space while you struggle to clear the Mystery Boxes.

Marble Sort Level 21 Step-by-Step Walkthrough

The puzzle here is strictly about the order of operations. You have exactly enough safe moves to open the Tunnels. Execute them perfectly.

Unlocking the Tunnels in Marble Sort Level 21

Your first taps must clear the immediate bottom frontline to satisfy the "3" requirement on those upper Tunnels.

1. Tap the two Blue blocks located on the frontline of the Top Tray. They will drop and immediately clear the two Blue bottom Boxes on the left side.
2. Your tunnel countdowns just dropped to 1.
3. Tap the frontline Black block on the far left. Tap the frontline Green block positioned perfectly above the bottom Green Box.
4. These marbles will flow into the active Black and Green bottom Boxes, clearing them instantly.
5. Both the left and right Tunnels will now open.
6. Leave the Red block alone. Do not touch it. It remains dangerous bait.

Revealing the Mystery Grid in Marble Sort Level 21

Clearing that first wave fundamentally shifts the board. You are now in the midgame.

1. With the front line cleared, the 3x3 grid of Mystery Boxes is now the active frontline. Drop the bottom row of these mystery blocks.
2. Your active lower Boxes have now shifted. You are looking at Yellow, Yellow, Green, and Black as your new targets.
3. Evaluate the colors revealed by the Mystery Boxes. If they reveal Yellow, Green, or Black, match them immediately to keep the board moving downward.
4. The right Tunnel is now dispensing Green and Brown marbles. You have a Green bottom Box active in column 4. Feed the Green marbles from the right side into it.
5. Watch your Conveyor Belt. As the Mystery Boxes reveal random colors, only pull down what perfectly matches the current active row of bottom Boxes.

Pushing Through the Final Layers in Marble Sort Level 21

Once the central mystery grid is cleared out, the board opens wide.

1. Your lower columns will start asking for Brown, Pink, and Light Blue Boxes.
2. The left Tunnel holds your entire supply of Light Blue and Pink marbles. Start pulling from the left flank to clear column 1 and column 3.
3. The right flank and the staggered far-right column hold your Brown blocks. Use these to clear the mid-layer of column 2 and column 3.
4. By the time you finally reach a Red bottom Box, you can safely drop that original front Red block that has been waiting patiently since step one. Keep your belt empty, match the revealed colors patiently, and the level clears itself.