The strength of the Ottoman forces is also estimated at about 15–20,000;[1] but inflated figures are common here as well. Numerous sources provide estimates of the size of the army as up to 60,000 including the Ottoman historian Şükrullah, who, writing in the 1460s, gives the figure of the Ottoman army as 60,000 in his Behçetu't-Tevârih;[2] alternately described as roughly half of the Crusader army.[3] The Ottoman force also included 1,500 Serbian heavy cavalry knights[4] under the command of Prince Stefan Lazarević, who was Sultan Bayezid's brother-in-law and vassal since the Battle of Kosovo in 1389.[5]