I don't want to be smart but spies even though they are expensive to run, will open every citadel door providing you have enough of them and they are good enough.
However, in cases like this, if you had two siege towers, at least one should reach the walls and get some of your troops on the walls. I have noticed a few times, the AI using ballistas and catapults to break down the walls so they would then get in through the cracks.
And of course in RTW, you can sap, which is very useful!