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.