I'd tend to agree with others that have pointed out siege engines as something that should perhaps be a special case. At the very least it would be important to have the siege workshop have some sort of store of them or allow them to be built in every region type. Otherwise switching from field battle to that big assault against the important city is going to be a way too long process with being able to add only one siege engine "unit" per day. In an army of 30 infantry units that's going to take a long time.
