The "Engine" of Minas Mor-go
This is how I imagine Minas Mor-go is propelled. The steering wheel is either on top of the gates or in the highest tower. When the wheel is turned, a giant basket of man-flesh dangling from the dungeon ceilings moves either left or right. The orcs, inside the dungeons, are continually rolling the wheels' axles. When the basket of meat moves to the left, the orcs to the right push further forward than the orcs to the left, because the right orcs are further away and want to be closer to the meat. Because the right end of the axle is now pushed forward, Minas Mor-go moves to the left. Speed can be managed either by dangling more baskets of meat or by whipping the orcs. As for brakes, we can just throw the orcs under Minas Mor-go's wheels.
I also assume we have something like a steam engine in there, too, where we can use the orcs as coal to fuel the engine. I suppose we don't have enough water to use the steam engine continually, and we'd use it only in emergencies or when we want to scare the orcs.