Arimaa/Introduction to Strategy/Camel Hostage
Strongest Free Piece
In the position at right, the gold elephant is holding the silver camel hostage. The camel is frozen, so it can't escape, and on any turn Gold could flip or pull it into the c3 trap. To preserve the camel, Silver must station his elephant next to the c3 trap. Silver can't defend with less than the elephant, because any other silver defender could itself be captured in c3, unless there were two defenders securely in place, which would be hard to maintain if the elephant were not one of them.
Herein lies the answer to the defensive conundrum of equal forces stalemating each other. Gold's elephant is tying up Silver's elephant and camel both, leaving Gold's camel unopposed as the strongest free piece. As long as the camel hostage remains in place, Gold can dominate the other side of the board. The silver camel obviously can't defend f3, and at present the silver elephant can't defend f3 without losing the silver camel.
One might be inclined to immediately advance the free camel, but advancing horses is much safer. Imagine in this diagram that instead of the horse, the gold camel were on a6. Silver to move could send his elephant to b6, setting the stage for an even camel trade, which would be a disappointing outcome for Gold. Since the camel captures would each use all four steps of a turn, neither elephant could come back and defend its own camel.
The gold horse on the west wing is actually not attacking so much as it is defending the c6 trap. The hostage camel keeps Silver's elephant next to the c3 trap, but it can still dart around as long as it is on c4, d3, or even c2 at the end of a turn. This mobility could allow the silver elephant to dislodge a small gold piece and gradually move it up. If the silver elephant could get a weak gold piece into silver territory, and Gold had nothing there to protect it, that gold piece would soon be lost while the silver camel would remain safe. The gold horses are prepared to defend the c6 and f6 traps, short-circuiting any such shenanigans by Silver. The gold horse in the west must keep some distance from the silver elephant, to avoid being frozen on c5 and then captured in two steps.
As long as the camel hostage is intact, Gold's best bet is to use his horse on the east wing to drag down one little piece at a time for capture in f3. This seemingly tedious strategy can ultimately decimate Silver's forces, if the silver camel and elephant both remain away from the action. Silver rabbits in the northeast are especially vulnerable, since Silver cannot hope to shield them all, and a rabbit cannot retreat backward. With the silver camel stuck in the west and the silver elephant committed to defending it, any available silver piece which tries to protect the f3 trap will itself be captured by the gold camel. On g3, the gold camel can deter silver horses from advancing, or capture them if they are reckless enough. The gold horses are the pieces which should operate away from home territory for the time being. Even if a smaller gold piece were to get pulled up, the gold horses could defend the c6 and f6 traps, and still deliver smaller silver pieces to the f3 trap. Depending on what Silver does, Gold may later want to advance his camel, but should keep f3 protected, and avoid offering Silver a camel trade.
Realizing that Gold is poised to take control, what should Silver do? In a situation like this, Silver needs to free his elephant while minimizing the material loss. This does not necessarily mean saving the silver camel—if Silver moves his elephant to b6, and the silver camel is then captured, Silver can pull the gold horse from a6 into c6, capturing it in return. This is a material loss for Silver, but he has contained the damage and freed his elephant for use wherever it is needed. Alternatively, if Silver still wanted to try to preserve his camel, he could unfreeze it with his horse and push the gold dog away, burrowing the camel so that it could not be captured in one turn—this is an option only because Gold has neglected to put a piece on b2, which would have blockaded any a3 hostage in place even if it were technically unfrozen. Even as things are, however, burrowing the camel would only be a delaying tactic by Silver, since the camel would hardly be free, and would still stand to be captured if the silver elephant left.
Whether or not one can get immediate compensation for a camel loss, it is crucial that one's elephant be freed from a hostage which gives the enemy the strongest free piece. The best answer to a camel hostage is to rotate out one's elephant. Two other pieces can defend the trap if they have support behind them. In this game, diagrammed at right, Gold is strategically lost because the arrival of the silver dogs has freed the silver elephant to roam, whereas the gold elephant must stay put to prevent an immediate goal. Suddenly the tables have turned and Silver's elephant is the strongest free piece.
This is the last and most subtle reason for the gold camel to stay home on defense when Gold has the silver camel hostage. Imagine that the gold camel were at home in the diagram at right, lurking on d2 for example. Once the silver elephant left the f3 trap, the gold camel could confront the new silver defenders. Silver would then have to scramble to prevent multiple captures.
The most active defense against a camel hostage is to bring up pieces either numerous enough or strong enough to contest both enemy traps. In this game, diagrammed at left, Gold can't make a capture in either c3 or f3, despite holding the silver camel hostage. The gold camel would like to help regain control of c3, but the silver elephant is well-positioned in the way.
Who has the strongest free piece now? The silver elephant is more free than the gold elephant, but it isn't totally free, because if it crosses back to the silver side of the board, the gold camel will smash across into the silver horses around c3. Likewise, the gold camel isn't really a free piece, because it needs to lurk around in order to keep the silver elephant committed to defending c3.
As it turns out, the gold horses are suddenly the strongest free pieces. Gold can't yet force a capture at home, but he can use both horses to take over an enemy trap (most likely f6), threatening captures and ultimately goals. Silver might then move his elephant to the defense of that trap, and further threaten one or both of the gold horses with capture, but in that case Gold will stand to gain even more than that in c3 and f3. Gold's upper hand traces back to the camel hostage.
These few positions are only scratching the surface of possible play when a camel is held hostage, but they should give an inkling both of how a camel hostage gives one an advantage, and of how it is the foundation of much deeper strategies.