A square is a formation of troops intended to resist attack by cavalry (as at Waterloo and many other battles) or by large masses of poorly disciplined and not very well armed melee infantry (as in the case of the British army fighting against the Madhi's men in the Sudan or the Zulus in the Zulu war). The square formation masses firepower in all directions and prevents the men in the square from being outflanked or attacked from the rear. The weakness of the square is that it is particularly vulnerable to both artillery and massed fire of muskets or rifles.

The Romans also used the square for the same general purpose and had an even more advanced formation called "the tortoise" to protect against showers of arrows and other light missiles. The square itself was somewhat vulnerable to such ranged attacks, presenting a target that could hardly be missed, so the tortoise was intended to remedy that problem by covering troops above and on all sides with their large rectangular shields. The problem with the tortoise was that it was tiring to maintain for long, and it made the formation less effective at attacking and moving about. It was a good way to approach closely to an enemy who were very strong in archers or slingers.