Arimaa/Positioning for an Attack
Note: For simplicity, many examples below will assume that Gold is attacking the c6 trap. Naturally, the same theories apply equally in all 4 quadrants.
This chapter describes how to best position one’s pieces for an attack while outlining the methods a defender will employ to thwart the attacker. Attacking an enemy trap has multiple purposes such as capturing enemy pieces, clearing a path for a rabbit advance, obtaining a space advantage, etc. Fortunately, it is easy to establish one set of rules for positioning pieces that will apply in most attacking and defending situations.
Positioning the Elephant 
The attacking elephant should usually remain centralized. Moving the elephant to a square such as b6 or c7 will usually lead to a failed attack or even a blockaded elephant, though there are a couple of exceptions discussed below. On the other hand, experience has shown that an attacking elephant on the d6 square is ideally placed to support an attack and disrupt enemy defenses while still remaining nicely centralized. As will be shown later in this chapter, the c5 square is an acceptable square for the attacking elephant but, in many cases the elephant is less effective on c5 than it would be on d6. If the attacking elephant temporarily moves away from the d6 square, the defending elephant should seize the opportunity to occupy d6 whenever possible. Not only is d6 a good square for the defending elephant, but it is also worthwhile to deny the attacking elephant the square for its own sake. Therefore, once an attacking elephant occupies d6, it should not relinquish the square until a better opportunity appears elsewhere (such as the e6 square during a double-attack).
The ideal situation in diagram 1: The elephant and the horse have taken control of the c6 trap with devastating efficiency. Notice all of the following features of the position:
- the silver elephant, usually the key defender against an Elephant and Horse Attack, is more than four steps away from the c6 trap.
- the silver camel, also highly effective against an Elephant and Horse Attack, is on the wrong wing.
- the a5 dog can reach c5 with da5e ra8s ra6s db5e, but Gold can easily push the dog to c4, with double-trap threats at c3 and c6.
- the defender cannot scatter all pieces away from the c6 trap and, even if Silver were able to do so, it would allow Gold to begin a highly potent rabbit advance such as Rd1n Rd2n Rd3w Rc3n.
- if the gold elephant were on the c5 square rather than d6 then the defender could simply defend with 2 steps such as he7s he6w while still being able to move the elephant towards the northwest quadrant with Rf4s eg4w.
Two important keys to this powerful attack are the combination of attackers on b6 and d6 that make it very difficult and time-consuming for the weak defenders to reach the c5 square (where they are also vulnerable to a south-push), and the lack of balance in the silver army (i.e. silver should not have all the strongest pieces on the same wing).
By contrast, the position in diagram 2 will turn into a disaster for gold with silver to move. Notice that Silver can immediately play ed5n ed6n ed7s hc7e, holding the gold horse hostage. As discussed in another chapter (provide link), a hostage that is frozen by an elephant on d7 is very difficult to protect. For example, if Silver plays ed6s hd7s on the subsequent move, the horse will be forked between the c6 and f6 traps. This illustrates both the danger of a hostage piece in the d- and e-files and the importance of having an elephant on d6. As an aside, Silver should not continue with the Camel and Horse Attack in the southeast quadrant until first holding the gold horse hostage on d7 - otherwise, Gold will likely retreat the c7 horse from danger while still having sufficient time to defend the southeast trap.
Exceptions: Although the elephant should generally not be decentralized during an attack, there are exceptions. If there is a silver camel on b6, then it could be worthwhile to temporarily place a gold elephant on b6 in order to push the camel south. This could eventually lead to a camel hostage near the c3 trap. Also, if gold has a powerful goal threat in the northwest quadrant then it may be worthwhile to decentralize the elephant and move it onto the 7th or 8th rank. Be very, very careful when doing so, however, as an elephant can easily become blockaded if it ventures into the corners. Use a weaker piece to support a rabbit advance on the perimeter whenever possible.
Positioning the Camel and Horse 
Controlling the b6 and/or c7 squares is vitally important for the great majority of Arimaa attacks. The horse or camel should normally be used to secure these important squares since rabbits and cats are easily pushed out of the way. A dog can be a useful attacker in some cases (see Elephant and Minor Piece Attacks), but a dog is not normally considered a “strong” piece for the purposes of securing a vital square in enemy territory. To see why control of b6 and c7 is so important, the reader should study diagram 3 from this game. Gold had planned to aggressively advance pieces on the west wing before gaining control of either b6 or c7. However, Silver was able to thwart this attack and now the gold elephant is pinned to d6 with no prospects for continuing the attack (other than a futile attempt to control c5). However, any piece that stands on c5 is at risk of being pushed to e5, creating a double-threat certain to result in a material gain for Silver. Eventually, Gold had to abandon the c6 square and suffered through an inferior position in the middlegame. It cannot be stressed enough that one should normally control vital squares first (elephant on d6 and strong pieces on either b6, c7) before threatening material or attempting to advance rabbits on the wing.
Once the attacker has firm control over key squares, it becomes possible to either flood the trap with rabbits, cats and dogs or, if the silver elephant is not in the vicinity, it may become possible to begin capturing material around the trap.
Positioning a Dog or Cat 
It is often helpful to bring in additional pieces after the key b6 and c7 squares have been controlled. By bringing a cat and/or dog towards the c6 trap, Gold may eventually be able to move the elephant elsewhere on the board while still maintaining control around c6 with multiple pieces. As explained below, cats and dogs are best placed on either c5 or b6 after a trap has been fully controlled. A dog is sometimes strong enough to secure the vital c7 square, but that job is often best left to a horse or camel. It is not advisable to place any pieces on d6 unless absolutely certain that they will not be pushed towards the f6 trap!
Positioning a Rabbit 
A rabbit can be a very useful piece during a swarm attack but they can often be a liability for the attacker if the objective is to capture material. In diagram 4 with silver to move, it might appear at first glance that silver cannot prevent the loss of material at the c6 trap. However, there is one "nuisance move" at Silver's disposal that will delay a capture: hd6s Rc5n hd5w hc5e. If Gold responds with Rc6e Mc7s Mc6s Db3n then Silver will be able to counter with dc8s cd8s Rd6e cd7s. Although Gold should be able to win material in this tactical melee, the game will not proceed as smoothly as initially expected. This loss of time by Gold may not be a decisive factor in this game, but in many open, tactical games it is crucial to prevent nuisance or delaying tactics by the defender. Notice that Gold would have been able to immediately win material if it weren't for the rabbit on c5. Due to the weakness of the rabbit and its inability to retreat, they are much better employed for goal threats and blockades rather, and should normally be kept at least two spaces away from an important trap.
Defending Against an Attack 
The most important consideration for Silver’s defense of the c6 trap is the placement of pieces. If the trap is not defended by strong pieces, or if the pieces are misplaced, then Gold will easily be able to gain control of the key squares. Virtually all strong Arimaa players will setup their horses on the b and g files. This will allow the horses to defend the crucial b6 and g6 squares by taking just one step forward. A silver camel is also an excellent defender for the b6 square, but Gold could easily respond in that case by attacking the f6 trap instead. The camel is excellent at disrupting an Elephant and Horse Attack and, therefore, it is often best to keep the camel on c7, d7, e7 or f7 so that it is flexible enough to defend both traps. This is an important point, so it should also be stated from the attacker’s perspective: Gold should not attack with a horse unless the silver camel is far away and/or blockaded from approaching the trap in question. For example, if the silver camel is on f7, pulling silver rabbits onto d7 and e7 will make it much easier to conduct an Elephant and Horse Attack.
Silver can defend the c6 trap by placing horses on both the vital b6 and c7 squares. However, this would leave the f6 trap without a defending horse, and the opponent should always be expected to attack the weakest wing. In practice, most players will defend the c7 square (and similarly f7) with a cat and then place additional pieces on b7, c8 and d7 to prevent a horse or camel from pushing its way onto c7 via the c6 square. With the b6 and c7 entry points well defended, Gold will have to spend time disrupting Silver’s defenses before commencing an attack. One possibility for Gold would be to push/pull a piece from d7 to d5 with the elephant on d6. This creates an entry point at c7 for a strong piece while also flipping an enemy piece into the center of the board.
The defender should always look for opportunities to take an attacker hostage or frame a piece on the trap square. These opportunities present themselves quite frequently. Note that if the attacking elephant is pinned to the c5 square while a horse is framed on c6, then it will be possible for the horse to be pulled to d6 where it becomes forked between the c6 and f6 traps. This is yet another reason that the gold elephant should occupy the d6 square during an attack.
Positioning After a Trap Is Fully Controlled 
After Gold has controlled the c6 trap, it is often advisable to advance some weaker pieces towards the trap while maintaining a strong piece on c7 and an elephant on d6. Eventually, Gold could have enough pieces around the enemy trap to allow the elephant to move elsewhere on the board. If so, Silver may attempt to regain control of the c6 trap at a later time; it is important for Gold to control the crucial square c7 with a strong piece such as a horse or camel. Since Silver will often counter-attack from the east, the c7 square will usually become more important than the b6 square. Silver will be reluctant to move a piece to c5 if there is a risk of capture at c3, and so c7 and d6 (if there is no elephant there) will be the key infiltration points. If Gold can swarm the c6 trap with many weaker pieces, it will become extremely difficult and time-consuming for Silver to ever fully re-capture the trap.
If Gold gains full control over the c6 trap, the enjoyable task of trapping material becomes possible! Since Silver will probably have many pieces on the two back ranks, Gold will want a strong piece on the c7 square for trapping pieces standing on b7, c8 or d7. If Silver scatters the defenders, leaving a gap on c8, then Gold should immediately take the opportunity to advance a rabbit towards the gap. Gold should not normally finish a turn with a gold rabbit standing on c6 because that will allow silver to blockade the c8 square without any immediate risk of capture (due to the rabbit blocking the trap).