Rams do not have any melee armor (in fact melee attackers get an attack bonus of +3), so any unit with a melee attack can destroy a Siege Ram rather quickly. The best ways to destroy them are with Mangonels, Mangudai, cavalry, or infantry. However, due to their high pierce armor and HP, they can withstand most ranged attacks easily. In fact, it's sometimes useful to add a few Siege Rams to an army for the sole purpose of soaking up enemy archer fire to protect the main force.