Minas Tirith? Most likely... Check out the two photos below and leave your thoughts (via the comments below or head over to the Messageboard)...
A wide shot of the "castle ruin" set.
The set up close (notice the pillars at the right of the wide shot--they're the same). Also, check out what seems to be an elf with his bow in the center.
"But the engines did not waste shot upon the indomitable wall... As soon as the great catapults were set, with many yells and the creaking of rope and winch, they began to throw missiles marvellously high, so that they passed right above the battlement and fell thudding with the first circle of the city; and many of them by some secret art burst into flame as they came toppling down." -- Return of the King, Chapter IV