if you insist on pursuing your plan , then , continue your march across the northern plains , climb those mountains , have this eavesdropping lad , luke here , pose as a krull refugee , employ him to get inside the walls of cromwell town , get control of the city , then signal that you have control of it by lighting a bonfire atop the wall above the city for one hour , then extinguish it for a few minutes , then relight it and keep it lit for an hour .