I'm sure someting like this has already been proposed. But I got to thinking about Massada, the ancient Jewish hilltop fort, and the huge ramp the Romans built to get close enough to threaten the actual fort. The fort was so strong it took months to build the huge ramp needed to threaten it.
http://homepage.ntlworld.com/alansven.ohman/page8.html
In Civ terms both the superpowerful fort and the ramp used to siege it could be considered Wonders. Elaborate, elegant super-citadels are certanly part of the fantasy world (Minas Tirith) so maybe there's a place for one in FfH?
But how to build a 'Wonder' that it really a giant siege weapon? Well, maybe we (i.e. u) don't have to. The game already includes the Trojan Horse. It's a unique unit, so it can fill the bill.
Caer Decayshus +100% to 125% Defense, +8 to +10
+1 or +2
Great Commander, I do not know if it is codeable, but I really like the idea that this thing would
reduce an attacking unit's chance to successfully Withdraw. If that's not codeable, perhaps it could simply be given the Malkim fireball tower effect.
cost, Stone/Marble bonus TBD. Perhaps requires Earth mana to build? There's all sorts of ways to balance out making one city a real mutha to assault.
The structure should have a strong
effect, under the theory that one thing that will persuade your average rural dweller to swear fealty to your city, is the owndership of the finest fortification ever built. Quite the persuasive criterion, when the local Orcs set out to alieviate population pressures caused by yet another baby orc boom.
And to provide the sheet of paper to threaten this rock, perhaps the Trojan Horse can be tweaked a bit? Here I'm a bit short of specifics. Though if it were possible for the units inside the Horse to attack with a +City Assault bonus, that might work.
Like I said, a super-fort is about as obvious an idea as go can get, but what they hey. Any interest in such a thing?