Arimaa/Camel Hostage

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To capitalize on this camel hostage, Gold must create a threat in the east.


The position at right is a basic example of a camel hostage. If the silver elephant left the trap, Gold could capture the silver camel by flipping or pulling it onto c3. For now, Silver can only defend with the elephant; any other silver defender could itself be captured in c3, unless there were two weaker silver defenders securely in place, which would require some work.

If Silver ever abandons her camel, the gold elephant should then pull it into the trap, finishing on c4 or d3, near the action. It is thus important that the gold cat remain on c2. The silver camel is not held on b3, as Gold must occupy that square to keep shared trap control and thus avoid losses at home. Gold does not occupy d3, as any gold piece on that square could be pulled away and perhaps threatened with capture.

For now, Gold's elephant is tying up Silver's elephant and camel both, leaving Gold's camel unopposed as the strongest free piece. The silver camel obviously can't defend any trap, and the silver elephant can only defend one trap at a time. The gold camel is thus a grave threat in the east.

Use of free piecesEdit

Gold now wants to create a second threat and force Silver to make a tough choice. In preparation, Gold has wisely advanced a horse ahead of his camel; until the silver elephant comes east, a gold horse cannot be threatened there. A material exchange is possible if Silver moves her elephant east and blocks the gold elephant's path to e6 or f5. A horse-for-camel trade would be acceptable for Gold, but an even camel trade would waste the hostage.

Advanced horses can also protect weaker pieces. To defend the camel hostage, the silver elephant need only finish each turn on c4, d3, or c2; the silver elephant could thus dart around the trap and perhaps dislodge a small gold piece, which might then be delivered to c6 or f6. The gold horses are prepared to defend those traps. The western gold horse must keep some distance from the silver elephant, to avoid being frozen on c5 and then captured in two steps.

Gold could now attack the f6 trap, but if he wants to play it safe, he can keep his camel at home and use the eastern gold horse to drag weaker pieces down for capture in f3 (remember, rabbits can't retreat homeward). As long as the silver elephant defends the silver camel, horses are the strongest silver pieces which could defend f3, but that would be difficult with the gold camel right there; any capture threat at f3 would likely overload Silver. With nothing to stop the gold camel and horse in the east, several silver pieces could ultimately be lost there; the silver elephant will have to come east at some point.

Silver to move could easily trade her camel for a horse: if the silver elephant moves to b6, and the silver camel is then captured, Silver can pull the gold horse from a6 into c6, capturing it in return. The silver elephant could then go wherever it was needed. The loss of a camel for a horse is nothing compared to what Silver could have soon faced if she didn't move her elephant. Alternatively, Silver could play hb5ws Da2e ma3s, unfreezing and burrowing her camel so that it could not be captured in one turn. This is an option only because Gold has left b2 empty; gold pieces on a1, a2, and b2 would form a phalanx, fencing in any a3 hostage. Burrowing the camel would not free it, but would buy Silver time, as Gold would then need two turns just to reestablish a solid camel hostage, this time with the camel on b2.

Gold to move can stop Silver from doing either of those things. To fence in the silver camel, Gold could simply advance the b1 rabbit to b2, but a better option would be to slide the a2 dog to b2, the a1 rabbit to a2, and the b1 rabbit to a1; that way, the dog isn't blocked in. On the fourth step, the a6 horse could move to a7, precluding an immediate camel-for-horse trade.

Note:
In Arimaa, the strength and function of a piece depend largely on which enemy pieces remain. For instance, if each side has lost a camel and a horse, the remaining horses are the unique second strongest pieces on each side, and so have essentially the role that camels had initially. On a depleted board, a horse hostage may amount to a camel hostage, and dogs may function as horses would in a camel hostage position. If a hostage does not give one an advantage in free pieces, it usually should not be taken.

Multi-piece defenseEdit

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The silver elephant has regained mobility, so Gold has no advantage from holding a camel hostage.

When the opponent has the strongest free piece, this must be changed before he can capitalize. When the gold camel is active, the silver elephant cannot perpetually defend a hostaged silver camel. Fortunately, it might not need to. While one's elephant defends an away trap, friendly pieces can safely advance toward it. This often allows one to rotate the elephant out of hostage defense, replacing it with two weaker defenders supported by other friendly pieces. On 63s of this game, Silver rotated out his elephant. Suddenly the tables have turned, and Silver's elephant is the strongest free piece; the gold elephant can scarcely afford to move, especially with the silver rabbits threatening goal. Gold could temporarily retake e3 or f4 with a pull-and-replace, but Silver could punish such a move.

A nearby strong piece can strengthen a hostage position. A gold horse or camel might have pulled a silver dog away from the trap, making elephant rotation more difficult for Silver.


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Gold has just freed his elephant, but still has a fight ahead.

On 28g of this game, Gold completed an elephant rotation just in time. Silver had a strong attack in the southwest, but the silver strength concentrated there made the northeastern rotation easier for Gold. Gold had no goal threat and no piece on f6, so the silver elephant had some degree of freedom. Meanwhile, Silver's western goal threat remained formidable; Gold's elephant rotation hardly sealed the game.


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This hostage position makes the eastern gold horse the strongest free piece.

On 13g of this game, Gold held the silver camel hostage and was strong on both wings, yet could not force a capture in c3 or f3. The well-placed silver elephant prevented any capture in f3, while also protecting the silver horses from the gold camel.

Who has the strongest free piece now? If the gold elephant left the c3 trap, Silver would soon wipe out Gold's southwestern forces. If the silver elephant left e3, the gold camel might soon dismantle Silver's defense of c3. The silver camel obviously isn't free, and the gold camel can't go anywhere without quickly being confronted by the silver elephant, which could safely leave e3 if the gold camel advanced. The silver horses are not free, as they are defending the silver camel and each other. The western gold horse is not totally free, as it is stopping a potential Silver swarm of c3, which could give the silver elephant more freedom.

The eastern gold horse is the strongest free piece; Gold can soon create an away threat that will force captures somewhere. The gold camel should stay in place for now, but the f2 dog could advance and be replaced by a cat or rabbit. The silver elephant might then go north and make multiple captures, but Gold could more than make up for that by moving his camel west. Even if both silver horses escaped, Gold could capture the silver camel and minimize his own losses, as Silver would have lost time getting her horses to safety.

This example and the next show that an elephant not directly defending a hostage can still be restricted by it. An elephant rotation does not always free the elephant; it may just tie up more material in the hostage position.

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Each side currently has an elephant, camel, horse, and dog tied up in this hostage position.

In this game, Gold's elephant rotation left the silver elephant blockaded. Despite this, neither side had a clear advantage in free pieces. The gold elephant is not actually free, as it is the only thing between its northeastern army and the silver camel. Silver might move her camel south to attack c3, but then the gold elephant could safely go south also, becoming the strongest free piece. Silver might do better to send the c2 horse east, where it would be the strongest local piece and could work toward a goal threat. Such a threat would be stronger if the second silver horse were free; having a friendly horse blocked in weakens a hostage-holder's position.

Of these four examples, the first camel hostage was quite weak, the second was not quite strong enough, the third was strong, and the fourth was unclear. If the defender frees his elephant in time to mitigate a second threat, he will likely have the advantage. If both elephants and camels are tied up, horses will likely decide the game.

Camel hostage valueEdit

A hostage-holder wants to create a second threat that will overload the opponent. By contrast, the defender usually wants to rotate out his elephant. This means that the two players will likely advance pieces on opposite wings. The outcome of such a race depends on the initial positions of the pieces. For Gold, a camel hostage will tend to be worthwhile if two strong gold pieces are already on the non-hostage wing, and Silver is not well-developed on the hostage wing. Ideally, the free camel and a friendly horse will work together on the non-hostage wing, and the other friendly horse will guard against a potential elephant rotation on the hostage wing. There are other possibilities, but the free pieces are always key, and there is limited time to maneuver them. If the hostage-holder cannot soon create a second threat, the hostage may be weak.

If Silver is not well-positioned for an elephant rotation, she might abandon her camel and seek compensation elsewhere. While creating a second threat, a camel hostage-holder will likely expose a friendly horse to capture; the hostage value is thus related to the value of a horse-for-camel trade. A camel is estimated to be worth a horse and cat as an initial trade, so one might say that a solid camel hostage is worth a cat. This should be kept in mind if one has to choose whether to take a hostage or do something else.

If Silver is well-positioned for a quick elephant rotation, a hostage might be of negative value for Gold. The purpose of a camel hostage is to tie down the enemy elephant and camel, thus freeing one's own camel and/or horse to attack. If the enemy elephant quickly becomes free while one's own elephant remains effectively stuck, this has backfired badly.

If the balance of forces is such that the hostage-holder does not have an advantage in free pieces, a hostage may be of negative value even if the defender is not poised for an elephant rotation. If the defender is reasonably strong on the other wing, he might be happy to have an elephant deadlock in which the enemy elephant is marginalized in its own home territory. Being advanced and centralized, the defending elephant might eventually join an attack on the other wing. If such an attack threatens goal, the opponent may not have time to capture the camel.

Rotation strengthEdit

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This is typical of a successful elephant rotation; a team of silver pieces has freed the silver elephant and cornered the gold elephant. The a4 rabbit ensures that the silver dog could step right back to c4 were it pulled to b4. The c3 rabbit blockades the trap; if c3 were clear, the gold elephant could push the horse to e3, perhaps threatening it in f3 if the overall position allowed for that. The gold elephant could try to go around c3, but this would free the silver camel to attack the goal line, and Silver would likely have time to defend against any threat in f3. If a gold horse or camel is nearby, Gold might hope to reassert ownership of c3, but the silver elephant could likely thwart and perhaps punish such a move.
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Here a gold rabbit blockades the trap. Unable to step backward, the rabbit is stuck on c3 as long as b3, d3, and c4 are occupied.

Silver does not yet have a strong goal threat on this wing, so Gold has some flexibility. Gold could push the camel to b1 and perhaps slide the c3 rabbit to b3, clearing space for further maneuvers. This is a possible advantage of holding a hostage on b2; the hostage could be buried if necessary.

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If the silver elephant left, the gold horse might occupy c4 or d3, and any nearby silver piece would be at risk. To rotate out her elephant, Silver must either blockade both c4 and d3 or remove the gold horse from the area. Silver might consider flipping the gold horse away, but Gold could occupy d2 and e3 to prevent this.
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The trap is clear, but the gold elephant can't move through it, as the trap currently has no other gold defender. The silver elephant has left the quadrant, but may still be nearby; how free it is depends on the rest of the board. If Gold could displace the silver dog or horse, the silver elephant might have to return to the trap to prevent catastrophic losses. Once again, the a4 rabbit ensures that the silver dog could return to c4 if pulled to b4. The a2 and b2 gold rabbits are also key to Silver's position; if Gold could clear b2, the silver horse would not be safe on c2, from where it could be pulled to b2. That would give Gold a double hostage and also allow a gold defender to occupy c2.

If the silver horse and dog are safe, this is a strong position for Silver. If either piece can be displaced, however, this could soon become a strong position for Gold, especially if the silver elephant is far away.

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A hostage held by a c2 elephant alters the usual alignment, since c2 is typically held by a weak piece. With the gold elephant on c2, b3 may be vulnerable. Here the gold horse may not be able to stop an elephant rotation, as Silver could occupy b3 and b4 if the gold horse stepped east. A hostage held behind the trap may therefore be weak, especially if the opponent is well-developed on the wing.

If d3 and d2 were unoccupied, the gold elephant could move to d3 and pull the silver camel to d2 as a central hostage. Silver can usually prevent such maneuvers.

Other hostage patternsEdit

Double hostageEdit

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A double hostage is stronger than a simple camel hostage.

On 16g of this game, the gold elephant took a double hostage. With three strong silver pieces tied up, Silver's options are quite limited. Unless Gold is reckless, Silver cannot hope to abandon the hostages and remotely make up for a camel and horse loss. With Gold's large advantage in free pieces, elephant rotation would also be quite difficult for Silver. Neither hostage can escape; the g2 horse is frozen, and the silver camel is blocked by a phalanx.

A silver piece on g4 could weaken this double hostage; if the silver elephant then left, any capture the gold elephant could make would land it on a square other than g3, so the second hostage could escape or be defended again after an exchange. Realizing that Silver could complicate things by stepping the g5 cat south, Gold should take care not to expose his camel to quick capture, as he might only get an even camel trade which would free and strengthen the southeastern silver horse. Silver might then have a formidable goal threat.

High hostageEdit

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Gold has an unstable but valuable high hostage. (Game)

A high hostage is held on the fourth or fifth rank. This is a relatively rare pattern, as it is tricky to maintain, but it does allow the hostage-holding elephant to remain centralized. The silver camel is a high hostage here; a gold horse and a phalanx of rabbits keep the camel frozen, but b4 is not secure in the long term. If Silver eventually unfreezes his camel from the side, the gold horse could then be lost in c6, so this is a risky hostage position for Gold. It may be easier to hold a high hostage on a depleted board, as Silver did in this endgame.

Central hostageEdit

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A central hostage may face capture threats in two different traps.

A central hostage is held on the d- or e-file. If the hostage-holder has good control of both home traps, a central hostage might turn into a fork: Silver can play ed6s Md7s cc8s df6e, and Gold cannot defend both c6 and f6. This hostage position is more tactical than strategic; Silver gave up a horse to trap the gold camel. If a capture cannot soon be forced, a central hostage may be worth little more than an ordinary hostage.