Well I’d smuggle in some Warlocks ahead of schedule. Park 'em in strategic but hidden locations. Then have some Rogues meet them. Summon the army on up.
Or more plainly a portal. Or flying mounts. Or maybe the demolishers were originally in place to shoot soldiers unto it. Who knows. Not really that important.
Don’t try to approach Azeroth with Earth logic. You’ve already put more thought into it than the developers have the second you do.