I am having a terrible time trying to find the key to the door underneath the windmill. I enter the cave in the marsh, go down the stairs and can see the rocky bridge to the other side where the passage has collapsed. The walkthrough, (which I really enjoyed on its own ) describes an unexpected, right-angle corridor. This I cannot find. Can you please help me.. I love this game and would love to finish it! Thank you

Hello, Ctaro
Did you enter the wind-mill ? Did you try all its possibilities, entering then by the big tree door to see what happened underground ?
If not, no way to find your... way
A walkthrough may be helpfull but, sometimes, lead you in a dead end by using short cuts. Nothing worth your own brain

For instance, you say

Do you mean you didn't try to pass this rocky bridge to see what is forward ? Is the passage collapsed yet ? Sure ?