They have some gigantic, amazing Star Wars sets, so I think we can dare to think big. Minas Tirith would obviously be the crown jewel of the Middle-earth sets. Up to twice as big as Helm’s Deep, but no bigger. It doesn’t have to have seven tiers. It just needs to be treated as a “castle” (rather than a whole city) and abstracted out in the normal way every castle has ever been. As long as it has the recognizable shape — rounded wall, spur of rock, Tower of Ecthelion — you can focus on play elements. Let it swing open so you can have a Houses of Healing room, a Houses of the Dead room, a Hall of the King. Do something cool with the gate.