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. In order 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 would itself stand to be captured in c3, unless there were two defenders, which at this point would be very difficult 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 allowing the silver camel to be captured. The silver horses must beware of the gold camel, though as we will see Gold's best option here is to keep his camel where it is, and use his horses to drag smaller pieces toward the f3 trap.
It might seem counter-intuitive that Gold takes advantage of having a free camel by advancing horses rather than by attacking with the free camel, but it turns out to be the safest way to play for advantage. Imagine in the diagram above that the gold camel on g3 switched places with the gold horse on a6. In that case Silver to move could send his elephant to b6, abandoning the silver camel to its fate, but taking the gold camel hostage at the same time. Gold could then capture the silver camel, but in doing so would use up all four steps of a turn, and thus not have time to protect his own camel, which Silver could then capture. This would be an even camel trade, which Gold is wisely not offering Silver.
The gold horse on the west wing is actually not attacking so much as it is defending the c6 trap. Silver's elephant is tied to the defense of the hostage camel, but it can dart around the c3 trap as long as it is on c4, d3, or even c2 at the end of a turn. This mobility is often enough that the silver elephant can dislodge a small gold piece and gradually move it toward c6. If the silver elephant could get a gold piece to c5 and Gold had nothing to unfreeze it or protect the c6 trap, Silver could then capture the piece in two steps and return his elephant to c4 in time to keep the silver camel safe. The gold horse on a6 is prepared to move to b6, c7, or d6 if necessary, protecting the trap and thereby short-circuiting any such shenanigans by Silver. This gold horse must keep some distance from the silver elephant, to avoid being captured while the silver camel remains protected.
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. While this strategy may seem tedious, it 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 back to its home row once it is pulled away. 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, which is best kept on g3 for the time being, to deny Silver a chance at an even camel trade. Gold might be tempted to send his camel after a silver horse, but that could expose the gold camel and possibly leave the f3 trap vulnerable as well. The gold horses are the pieces which should be operating away from home territory for the time being. Even if a silver horse managed to pull up a smaller gold piece, 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 take care to keep f3 protected, and avoid offering Silver an easy camel trade.
Realizing that Gold is poised to take control of the game, 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 would not be threatened with capture 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.
If Gold does not present a horse in a way that allows Silver to capture it in exchange for the hostage camel, then the only active defense lies in bringing a wave of pieces up to the hostage trap in hopes of freeing the elephant from its defensive duties. 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 been turned and Silver has the strongest free piece, i.e. the silver elephant.
This is the last and most subtle reason for the gold camel to stay home on defense when Gold has the silver camel hostage. If the gold camel were at home in the diagram at right, lurking on d2 for example, it could threaten whatever pieces Silver brought up to defend f3. The silver elephant would have a tough fight to try to prevent the loss of a piece in c3 without losing control of f3. The tactics can get complicated, but with the camel helping out, Gold should be able to make captures somewhere, or at a minimum keep the silver elephant from straying far.
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.