I have trouble understanding how promotion prerequisites work. Civilopedia lists Accuracy I and Barrage I as being prerequisites for Volley, Cover and Siege.
However, all my trebuchets only needed Barrage I to access Volley and Cover I (I chose Volley). Now that one of them is ready to receive another promotion, only Cover I is available, Siege is not.
So I simply don't understand how the whole thing works.
Best Answer
In cases where two prerequisites are listed for a promotion, typically you only need one or the other.
In cases where a promotion unlocks multiple other promotions, you can always pick the ones you turned down at future level ups.
However, in Gods and Kings, Siege was made into a Melee and Air only promotion, with the Air variant having 3 ranks. Oddly enough, Siege is not available to siege units, such as artillery in the G&K expansion. To compensate, most siege units default bonus versus cities is 200%, up from 20% in the base game.
Therefore, you're not missing the promotion by taking Cover or Volley - it simply does not exist.