Javelin cavalry.

For a siege? You bet. Use a ram to break down the gate. 99.9999 percent of the time, knights or armored infantry will be there. Javelin cavalry will take care of those.

Cavalry are better for this than foot javeling throwers because jav-cav can throw on the move. Foot cav can't throw while skirmishing away. Therefore, jav-cav lay down a continuous hail of missiles that quickly clear the way.