An all but impenetrable formation of pikemen. Developed by the Swiss in 1444, it consists of lots and lots of 17 foot pikes arranged in a square, with ranged units (bows, and later arquebuses inside it. The normal tactic against pikemen until the pike-square was introduced was to flank them, because while they were excellent against a frontal attack, they were not particularly mobile units. When they formed themselves into a square, however, there's nothing to flank, as the pointy bits are coming from all sides.