Malbim: These are great walled cities, in which were officers and the king's army. They were a shield for the Jews and actively helped them; their afflicters feared to approach them or stand against them. The Amim did not know of the sealed letters. They did not whether the king permits to fight the Jews and kill them. Even though the first letters were not canceled, they feared the Jews, who had the upper hand due to the king's edict.