Arimaa/Trap Control

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At the outset, each player essentially has full control of his two home traps. Each elephant can quickly share control of one away trap, but as long as a home player can occupy two key squares of each home trap, having the enemy elephant next to one is not a problem. After advancing the elephant, one might try to get a friendly horse onto a decentralized key square of an away trap (e.g. b6 or c7 when attacking the c6 trap). If that is accomplished and there is nothing to immediately displace that horse, the opponent may struggle to retain even shared control of his home trap.

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The b2 horse effectively gives Silver shared control of the c3 trap. (game)

Sharing control of an away trap will limit the opponent's capture threats, which will in turn allow one to play more aggressively. As long as one adequately defends an away trap, no friendly piece can be captured therein. This allows more pieces to advance toward it, perhaps bringing one closer to taking full control of that trap, or to sharing control of it even if the friendly elephant leaves. If the opponent can't afford to give up the trap totally, he must commit material to its defense, leaving his forces thinner elsewhere. If one does get full control of an away trap, several quick captures may be possible, since an enemy trap usually has plenty of enemy pieces nearby. Once space has been cleared, by captures or by enemy pieces scattering, a strong goal threat may be possible.

On the other hand, one attacking for trap control might soon find the tables turned on him. If too many pieces advance too quickly, the home traps may be vulnerable to a counterattack. Moreover, advanced pieces may end up hostaged, framed, or blockaded. Even if the trap defense appears too weak for that, stronger defenders may arrive later, if the whole-board fight allows. Attackers may even end up blocking each other. Rabbits often advance to support an attack, but since they can't retreat, the defender might use them against other attackers. The attacking side might then be weak on the entire wing, with any attack blocked and fewer pieces defending home territory.

In the position shown, a silver horse has penetrated enemy lines in the opening, giving Silver a long-term trap control advantage. Although Silver has no immediate way to take complete control of the c3 trap, Gold has no direct way to deal with the intruding horse: the silver elephant would stop the gold camel, and Gold cannot afford to decentralize his elephant just to deal with a horse. If the gold elephant goes east, Silver can safely advance more pieces in the west.

Deadlocked Traps[edit]

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There is an elephant deadlock at f3 and a camel deadlock at c6, but Silver to move can end both and come out ahead.

A trap is deadlocked when its strongest gold defender and strongest silver defender are equal. Elephant deadlocks are common, as a hostage, frame, or trap control fight can easily tie up both elephants. A camel deadlock would normally be precarious. In the diagram, the elephants are deadlocked in a horse frame in the southeast, while the camels are deadlocked in a horse hostage in the northwest. This is not a stalemate, however. Silver to move can put her elephant on c4 and dog on d5, which instantly gives up the silver horse but also dooms the gold camel. Two deadlocks can be broken at once.

When an elephant breaks a camel deadlock, or any piece breaks a deadlock of weaker pieces, the side breaking it might soon own the trap. If the camels are deadlocked in a frame, and the pinned camel is dislodged by the enemy elephant, the framed piece will be gone, and the formerly pinned camel could soon follow. If instead the pinned camel's friendly elephant breaks the deadlock, that elephant must relieve the pin and extract the formerly framed piece before any capture can occur. The former maneuver, in which captures could begin immediately, would tend to be stronger than the latter.


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Gold will lose a horse for abandoning c6; Silver will lose a dog and some rabbits for abandoning c6.


When both sides commit an elephant to the same trap, neither can capture anything in that trap. Sometimes, a player willingly allows a friendly piece to be framed or hostaged, due to the space advantage of deadlocking elephants around an away trap. Pieces can safely advance toward a trap defended by the friendly elephant; by contrast, the home elephant would have to leave the deadlock if it wanted to ensure safe advances of its own pieces.

When there is a northwestern elephant deadlock, it is tactically safe for gold pieces to advance in the west. Before and after diagrams illustrate the effect of rabbit advances on such a deadlock.

In the before diagram, the gold elephant does not want to abandon c6, because Silver would capture the b7 horse. Likewise the silver elephant does not want to abandon c6, because the gold horse and elephant together could force the capture of any silver piece near the trap. This is the essence of an elephant deadlock: either side will suffer for leaving.

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Gold will lose a horse and some rabbits for abandoning c6; Silver will lose a dog, some rabbits, and the game for abandoning c6.

In the after diagram, the advanced gold rabbits have further raised the stakes for both sides. If the gold elephant abandoned c6, Silver would then clean house. If the silver elephant abandoned c6, a gold rabbit would soon reach goal. With so much now at stake in the northwest, the deadlock has tightened considerably.

In advancing three rabbits in the west, Gold may have figured that the threat of an eventual goal outweighed the risk of eventually losing a few rabbits. That is often correct, but the whole board must be considered. If the silver camel advances in the east, it can soon cause trouble unless the gold elephant leaves the deadlock. The likely result would be a three-for-one trade, which would strongly favor Silver on this already depleted board.

One whose elephant is stuck in a home deadlock must force the opponent to do something other than work that deadlock to his advantage. One unprepared to make a strong second threat should avoid getting into a home elephant deadlock; for example, taking a hostage could backfire if one is not already poised for an attack on the other wing.

Contested Traps[edit]

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All four traps are contested, since each strongest local piece faces multiple weaker pieces which protect each other.

A trap is contested when one side has the strongest local piece, but the other has multiple weaker pieces which protect one another from capture.

In this game, diagrammed at right, all four traps are contested. Until one side can establish total control of some trap, no captures will be possible. Whoever gets ownership of a trap first will start capturing pieces, gaining a huge advantage unless the opponent can quickly respond in kind.

Multiple trap control fights essentially amount to a positional race, as opposed to a capture or goal race. One poised to lose control of a home trap must either defend that trap or counterattack somehow. Here, Gold can soon take over f6 or perhaps c6, unless the silver elephant goes north. If the silver elephant left the southeast, however, the gold camel could either go west and help wipe out Silver's forces around c3, or go north and help attack f6. Confronting either gold horse is thus not a viable option for Silver. The western gold horse would stop any Silver swarm of c3, so that leaves an attack on f3 as Silver's best choice. Even then, however, Gold will likely get control of f6 first, and advance more pieces toward it.

Each local fight must be considered in the global context. When a piece crosses quadrants to attack or defend a trap, the area that piece left might be affected.

Losing control[edit]

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The gold camel must take control of the f3 trap. (Game)

In the position at left, Silver has deadlocked both of Gold's home traps. The a3 horse will stop Gold from hostaging the b3 camel. Both gold horses are currently blocked in.

If Silver can maintain shared control of both c3 and f3, then Gold's position is hopeless, but if Gold regains full control of either home trap, any silver piece near that trap will be at risk. If the gold camel can re-assert gold ownership of f3, Gold might then attack f6.

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Gold shares control of f6, but Silver has taken full control of f3. (Game)

In the position at right, Gold attacked f6, but Silver counterattacked f3 and now has full control of it, whereas Gold only shares control of f6. Gold to move can't even defend f3 in one turn, as silver pieces place five steps between the gold elephant and e3 or f4. Gold could now try attacking c6, but the silver elephant could defend it without giving up silver control of f3. The gold elephant must contest the f3 trap as soon as possible, or else Silver will clear space for a goal.

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Silver owns no traps, and can't even move pieces through her home traps. (Game)

In the position at left, most of Gold's army has advanced into Silver territory; Silver is severely cramped, with silver pieces unable to move through their own home traps. The silver elephant is currently stuck contesting f6, and thus the gold elephant can dominate the west. Gold defends all four traps, and Silver has nothing close to a capture threat anywhere. Gold pieces block nearly anything Silver might try.

Gold's one potential weakness is his thin home defense. Gold should keep an eye on Silver's h-file horse and dog, which might come down and try to start something in the southeast.