Isn't Minas Tirith just on a plane?
One of the many things they got right in the LorR movies was Minas Tirith. Minas Tirith WAS built on a plain yes. But It was backed up against a mountain and watched over one of the major crossing sites on a river that prevented the forces of Mordor and the Easterlings from easily crossing. One of the reasons Minas Tirith was so damn powerful was it's advantageous positioning and the fact it HAD to be.
It's a 7 tiered city, cut right into the mountain side. Made it nearly indestructible, gave it a huge advantage in sieges, and generally made it bad ass. It was built to have passes through the back for escape and supplies if necessary, and generally making it impossible to simply "wait out" even if that was something the orcs never would have considered anyway.
Anyway. For a minas Tirith city, you need 7 tiers I believe (or was it 9), access to each tier should be a bunch of ramps and gates and alternating on which side. Finally. Catapults. Lots of catapults all along the walls on the edge of each tier.
Finally, I'm about to post my current project, is to have a fortress with only one access point, a massive bridge which has entire areas that can only be accessed by bridge... Currently going pretty well with a few accidents, but I'll have those sorted out soon. The amazing thing is the two simulated water falls on either side of it! One is supposed to be a fishing pond (but that's not going so well >.<) and the other is actual water. I'll post another picture for next winter when they both freeze up solid. It's incredibly cool looking.
Both waterfalls drop into a chasm, so no flooding problems either :3
edit: By the way, while I'm here. Any ideas on getting the fishing reservoir/fall to work? I have tunnels there's a channeled out hole that should drop water directly there (without the filtering effects of stairs) is that not enough? Is there anyway to get fish to come down from the brook above and stay?