[Knight of Gondor]

I thought about posting this on The Movies, but it's probably more of an N & N question. The orc-forces had the Oliphants with them (actually, I think they came with the Haradrim, but whatever) to join in the siege of Gondor. But what good are the Mumakil on the plains? If there are ranks of troops, then yeah, they are deadly. But they came intending to assault the mighty walls of Minas Tirith! What, are the Oliphants going to charge the walls? Can the troops in the battle towers jump from the towers over the walls? It seemed a little impractical for the tactics they were planning. Thoughts?