The Wikipedia article (link below) has more information on this military formation.
A sheltron (also sceld-trome, schiltrom or shiltron) is a compact body of troops forming a battle array, shield wall or phalanx.
The term sheltron is obsolete, but is most often associated with Scottish pike formations during the Wars of Scottish Independence in the late 13th and early 14th centuries.
The term dates from at least 1000 AD and derives from Old English roots expressing the idea of a “shield-troop”. Some researchers have also posited this etymological relation may show the schiltron is directly descended from the Anglo-Saxon shield wall, and still others give evidence “schiltron” is a name derived from a Viking circular formation (generally no less than a thousand fighters) in extremely close formation, intended to present an enemy’s cavalry charge with an “infinite” obstacle (that is, a perimeter horses refuse to breach). Matters are confused by the fact that the term in Middle English could clearly refer to a body of soldiers without reference to formation, including cavalry and archers. The first mention of the schiltron as a specific formation of spearmen appears to be at the Battle of Falkirk in 1297. There is, however, no reason to believe this is the first time such a formation was used and, indeed, may have had a long previous history in Scotland, as the Picts used to employ spears in block formation as the backbone of their armies.
Never for one moment had he regretted marrying her, but she could break his defences the way he used to break the schiltrons.
Source: The Company by K J Parker.