Damn, I have to replay again....
Maps like 6424 are, in my opinion, very difficult to complete by simply trying things out without studying the map editor and creating at least a general plan of action. At best, you'll have to replay for a couple of days. At worst, you'll lose completely. Moreover, on such maps, there are hidden points - "events" that cannot be detected with the naked eye, but which are important, for example, they contain a quest artifact.