The Byzantine Empire in the 9th to the 11th centuries used highly sophisticated combined arms tactics, based around hollow infantry square formation. The infantry square, consisting of pikemen and archers, acted as a base of operations and refuge for cavalry by forming what was essentially a mobile fortified camp. Cavalry would ride out of the square through gaps in lines to exploit opportunities for attack and retreat the same way if the situation turned against it. The infantry square described by Nikephoros Phokas consisted of 12,000 men, who were deployed in 1000-man taxiarchies, which were separated by intervals wide enough to admit a dozen cavalrymen riding abreast to enter or leave the square.