There is a warp in Whomp's Fortress that makes going up the fortress a lot easier. Go until you reach the area where a Thwomp is pounding the ground. There are several regular boxes and a red ! box there as well. Stand in the corner between the wire wall and the fortress wall. The game will warp you to a ledge coming out of the fortress, next to a large pole. You can also warp from the ledge to the area where the Thwomp is.