In World 5 at the haunted house level. Proceed through level while avoiding monsters until you reach the platform above the lone door. When you see three blocks, jump above them so three new blocks appear. Jump off the near wall until a monster appears. Kill it, then. from the new blocks, jump again. Climb the vine that appears to reach a door. Continue through the level and exit the door, then finish the level. The cannon level will now be unlocked at the world map. Enter it to find a cannon that reads "World 8", which leads to World 8.