Currently a Great Eagle can, by itself, raze a full-size city 30% of the time.
This occurs when the city 'Raze Defenders' have 0-1 ranged units, or they have two but roll badly in the simulated battle. Since the rest of them - typically swordsmen or cavalry - can't kill the flying Great Eagle, the game decides the Great Eagle is strong enough to raze the city.
Anyone with a ruleset can make other buildings harder to raze by boosting their Avenger lists, but city defenders are autogenerated. I think you should be able to make them stronger with your hex editing, though I don't know how difficult it would be.
In AoW2 there is a parameter for the gold value of city defenders, which can be accessed via the MPE mod editor.
I think it's the same for AoW1's city defenders and dungeon guards, the guards generated from the unit list are limited by a gold value. So increasing that gold value with be very desirable!
Post edited May 24, 2020 by southern