There was mention of justifyable use of catapults/projectors on page 2 of this thread. It was to knock down gates in heavily populated areas, such as Greece. I have an alternative suggestion:

The projectors cost how much again? How many spies could you train for the same amount and keep in the field? Instead of a mammoth projector unit sitting around most of the time, send hordes of spies into Greece and swarm the cities. Some of the spies will get in, some will be killed. When you get there, there is a good chance that the gates are open - all of them. The spies can then be reassigned for scouting, etc.