A schiltron (pronounced SKILL-trun, sometimes spelled "schiltrom") was a formation used by Scottish infantry during the High Middle Ages, reputedly invented by William Wallace. It was a simple formation: a mass of foot soldiers armed with long spears or pikes* formed into a ring, giving the appearance of a hedgehog or porcupine. Schiltrons could be rather small (50-100 men) or rather large (several thousand men); large schiltrons could be several ranks deep and may conceal archers and/or cavalry in their centers.

This formation was used frequently during the Scottish Wars of Independence, most notably by Wallace (unsuccessfully) at the Battle of Falkirk (1298) and by Robert the Bruce (very successfully) at the Battle of Bannockburn (1314). The tactic was also used by Flemish spearmen fighting the French at the Battle of Courtrai in 1302. Schiltrons were primarily a defensive formation, perfect for repelling the charges of mounted knights. The downfall of the formation was its mobility (or lack thereof), it being difficult to maintain a coherent wall of pikes at more than a slow walking pace. This made the schiltron vulnerable to concentrated archery fire, a weakness that Edward I exploited at Falkirk. By the mid-14th century the schiltron had become obsolete, especially with the English realization of the longbow's effectiveness, as demonstrated at Crécy.

By nature, the schiltron is similar to the Greek and Macedonian phalanx -- both utilized lightly armored spearmen arranged in a tight, defensive formation. The schiltron differs from the phalanx in three major ways:

-- Phalanxes were ordered in ranks and files, whereas a schiltron more resembled a mob with spears than an ordered military formation.
-- All the pikes in a phalanx pointed in one direction (preferably at the enemy), while the pikes in a schiltron pointed in all directions.
-- Greek hoplites carried shields, while Scots foot soldiers rarely did.

* - Ideally the men forming the schiltron would use pikes, but Scots infantry often were forced to use pitchforks, long axes (precursors to the Lochaber axe) and long pointed sticks.

The Schiltron also happens to be a vehicle in the BattleTech universe. It is a wheeled AFV used by the Inner Sphere (circa 3060), typically armed with an assortment of missiles and lasers.

The Battle of Falkirk and the Execution of Wallace - http://www.scotwebshops.com/history/features/falkirk/
Schiltron - http://www.chaosmarch.com/spheretech/vehicles/3060/schiltron.asp