Baron knights attract liegemen to your ranks at regular basis when reside in their homeland. The number of liegemen joining is equal to the number of villages around the castle. Normally liegemen are light spearmen, but if village is wealthy enough, it will equip and train heavily armoured squires instead. 
Note however that if you raise militia in a village, then its wealth is diminished. Thus you can only choose either militia or squires, but not both at the same time.