Corridor Strategy Guide

Openings, wall tempo, blocking patterns and endgame races - the tactics that decide games between strong players.

The one number that decides every Corridor game

Corridor - often called wall chess - is a race with interference. At any moment each player has a shortest path to their goal row, measured in steps. Subtract your opponent's count from yours and you have the position's whole evaluation in one number.
If you are on move and your path is shorter or equal, you are winning the pure race. If it is longer, you need walls to fix the difference before you run out of them.
Recount after every wall, yours and theirs. Most losses come from spending a wall that cost the opponent two steps while costing you three.

The strongest generic opening is simply to advance up the centre file. Walls placed on move one rarely pay for themselves, because the opponent has eight open files to route around.
The centre-file march keeps every detour cheap: from the middle you can swing left or right for the same cost, so early walls against you lose value.
A useful practical plan: advance three or four squares, then place your first wall only when the opponent commits to one side of the board.
Against players who open with an immediate wall, ignore it unless it costs you two or more steps. Answering a one-step wall with a wall of your own trades your currency for theirs at a bad rate.

A wall costs you a full move. So a wall is only good when it costs your opponent at least two steps, or when it costs one step and also removes their best future placements.
Tempo works in both directions: while you place, they run. Placing three walls in a row hands the opponent three free steps, which is often the whole margin of the race.
The cheapest good walls are the ones that force a detour around an existing wall. Chain your walls so each new one extends a barrier instead of starting a fresh one.
Placing a wall that costs the opponent zero steps is worse than passing: you lose a move and a resource at the same time.

You get ten walls and no more. Treat them as a budget spread over the game rather than an opening weapon.
If you spend your last wall while the opponent still holds three, the rest of the game is played on their terms - they can shape every corridor you must walk through.
A healthy rule of thumb: never fall more than two walls behind unless each wall you spent bought at least two steps.
Keeping one wall in reserve for the endgame is worth more than most players think - a single late wall can flip a one-step race.

The staircase: alternate horizontal and vertical walls diagonally ahead of the opponent so each one adds steps to the same detour.
The funnel: leave exactly one open channel and place your remaining walls where they lengthen that channel, not where they threaten to seal it (sealing is illegal and the placement will be rejected).
The side pin: when an opponent hugs an edge, walls there are unusually strong because they have no room to route around on the outside.
The double-purpose wall: the best placements both slow the opponent and shorten your own detour options later. Look for walls that sit beside your own route, not across it.

Once walls run out, the game is arithmetic. Count both shortest paths; the player on move with the shorter or equal count wins.
If you are one step behind with a wall left, the wall must cost the opponent two steps - a one-step wall only ties, and being on move decides ties.
When you are ahead in the race, avoid squares where a single wall costs you two steps. Walk down the file with the widest escape options, even if it looks less direct.
When you are behind and out of walls, take the route with the most branch points and hope the opponent commits their last wall early.

Wall-spamming the first three moves and then losing a pure race by two steps.
Placing a wall that shortens your own path only in theory - verify it does not force you into the same detour.
Ignoring jumps: an opponent standing directly in front of you can be jumped, which sometimes gains a full step for free.
Forgetting the no-sealing rule and building a barrier you cannot legally complete, wasting the walls already spent.