The Army of Flanders had been built upon the concept of the Spanish "tercio", a pike-heavy infantry formation that well suited the nature of warfare in the Netherlands. The large areas of flat ground, the "platteland", was criss-crossed by rivers and drainage channels, dotted by numerous towns and cities well placed to dominate the surrounding landscape, increasingly defended with polygonal fortifications. Siege warfare, rather than set-piece battles, dominated the Eighty Years' War, especially in the 16th century. Away from the major sieges, the war took on an almost guerilla style of small engagements and skirmishes, with much of both the Army of Flanders and the Dutch forces dispersed across the countryside; in 1639, for example, just under half of the Army, then 77,000 strong, was distributed across 208 small garrisons. This pattern reflected the Dutch disposition as well. Siege warfare was extremely expensive, both in terms of casualties and money. In 1622 the siege of Bergen-op-Zoom cost Spinola 9,000 men, whilst the siege of Oostend in 1601-4 cost the Army of Flanders 80,000 in casualties. The siege of Breda during 1624–5 was so expensive financially that the advance had to pause through 1625 – no more money was available to exploit the success.