How did Alauddin Khilji expand his empire?

After conquering the bulk of northern India, Alauddin Khilji sent an expedition to Deccan under Malik Kafur. The kingdoms of Devagiri, Warangal, Dwarasamudra and Madurai were conquered but not annexed. Alauddin Khilji knew that it was difficult to control these distant lands from Delhi and also he was only interested in acquiring the wealth of these Deccan kingdoms for the maintenance of his huge army. That is why he forced the rulers of Deccan to accept his overlordship and to pay annual tribute to him.