Foxhold: Sudoku as Tower Defence

Sudoku and tower defence, joined

Tower defence usually goes like this: you buy towers, place them beside a path and watch. Sudoku usually goes like this: you think for as long as you like, and nobody hurries you. Foxhold takes from each what the other is missing. A tower is not bought here, it is deduced. And when you are stuck, you do not lose money, you lose quiet.

That produces a tension neither side has on its own. A sudoku has no order to it: whether you place the 4 in the top left or the bottom right first makes no difference. In Foxhold it does. The left tower belongs to the left third of the grid, and while that third is still empty, enemies walk past it untroubled.

The clock never stops. Depending on the difficulty you get roughly 28 to 35 seconds before the first wave and 15 to 19 between waves. Both are more generous in the first thirty levels, so the start does not fail on the controls. You may place digits at any time, including in the middle of a wave: the pressure comes from time moving on, not from the game taking your keyboard away.

How digits turn into towers

Three towers stand above the grid, one for each vertical third: left, middle, right. What you do in the grid takes effect directly above it. Two quantities decide:

• A completed block raises the level. Each third consists of three 3x3 blocks. For every block you finish, the tower above it gains a level and its damage jumps visibly. From level 0 to level 3 that is more than double.
• Every correct digit makes it faster. The tower's rate of fire follows how full its three blocks are overall. That is why the eighth digit already does something, not just the ninth one that closes the block.

A sudoku commonplace turns into a tactical question. A block holding seven digits is not just further solved, it is worth more: two cells separate it from the next tower level.

The rules in short

1. Placing costs nothing. There is no currency in Foxhold. The only scarce quantity is the time until the next wave.
2. A mistake pulls the wave closer. A wrong digit costs two, three or four seconds of your breather, depending on the difficulty. It stays as a shaky tower, fights through the current wave and then disappears, leaving the cell free again. If the penalty lands during a wave, it is taken off the next breather.
3. Three hints, no more. A hint fills one cell correctly and costs three times what a wrong digit costs. After that it stops, otherwise you could simply fetch half the puzzle.
4. Calling early pays off. Ready sooner? Then pull the next wave in yourself. Nothing is handed to you for it, and that is the point: the run gets shorter, and short runs score higher.
5. The grid ends the run. While cells are free, more waves keep coming, sixteen at most. Once the grid is full, the finale is announced and the boulder rolls in. Finish early and the boulder is weaker.
6. Only won runs count. If the den falls there is no result, just as a half solved sudoku is no result. Your den takes 22, 20 or 18 hits, depending on the difficulty.

Who is coming

Six kinds of enemy walk the lane. They differ in three things: how much they take, how fast they move and how much it hurts when they arrive. There are deliberately no special effects, because those always hit the player who was losing anyway.

• Crow. The basic enemy. Comes in numbers, falls quickly, costs one hit.
• Swift. Little health, almost twice the speed. It is past your towers before they have fired often enough.
• Armoured. Slow and tough, takes a good three times what a crow takes and costs three hits.
• Mistwalker. The quick middleweight: more health than a crow, more speed than an armoured one.
• Blotter. Tough and sluggish. It ties up your towers while the rest of the wave walks through.
• Boulder. Comes in the finale only, once, announced. Bring it down and your den stands.

Foxhold at Sudoku Sagax

Foxhold has three difficulties with 800 levels each. They differ not in the strength of your towers but in how much your den can take (22, 20 or 18), how fast the enemies grow with every wave, how long the breathers are and what a mistake costs.

On your first run Nona walks you through a short introduction. In a few minutes she explains the den, the lane, the towers and the waves, and you can repeat it any time from the level list.

Every run is recomputed on the server, move by move, from difficulty, level and the action log. That is the only reason the leaderboard is worth anything. Foxhold keeps its own list and does not feed into the sudoku scoring, but it counts towards your daily streak like every other puzzle type. There are four trophies of its own: Den Fox for the first win, Unbreached for a run with nothing getting through, Own Light for a run without a hint, and Bulwark for a win on hard.

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Frequently asked questions