When you are a captive after "kidnapping" Marle, wait the three days instead of rushing ahead. From the execution room turn right, go up and keep going until you reach a cell with a hole in the wall, go out this hole and climb down the wall. keep going until you see another entrance. Explore this room until you see a black hole in the floor. Go down by pressing the A button. There will be a lot of prizes for those who had the patience to come here. go back up the same way to get back into the prison.