Dear Student,
Delhi was attacked by Mongols twice after 1219. This forced the two rulers to mobilise a large standing army in Delhi because the attacks increased during the reign of Delhi Sultans.
They did Manage such a huge system by collecting Tax from LAN and rivers Ganga and Yamuna. The tax was 50% of the peasant's yield. The payment was done to the army by cash and soldiers buy the supplies from merchants in Delhi. Prices were also controlled by Alauddin and he used to survey the prices.
Regards,