You see! And who said discussing books wasn't fun!
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As I see it, Tolkien's 'Middle Earth' is created with great attention to deatail. There are possible flaws but they both have arguments for and against.
I'd like to point out that reference to the film here, is not much use as there were many changes in it, though most small. A large one being good ol' Tom Bombadil. One of my favourite characters from the book, who even had a book written about him, was discluded from the film and so it does not provide a true argument when referred to.
The whole argument about building a wall across the secret passageway is also pretty much invalid. For a start it's supposedly secret and so Sauron would not think that anyone would know about it, and he certainly would not expect the one creature that did know about it, to lead the ring bearer to it. Then there was Shelob. If memory serves correctly, from the Simarillion, we learn that Shelob was spawned from a creature which once defeated Sauron but let him go. Consequently, it would not be thought that anyone would be able to get past her. Then, there is the orc tower. Through which anyone must pass, before walking throught the whole of Mordor itself. What I'm trying to point out is, LotR is a book, and as with most books; things happen through circumstance and it was only through almost unbelieveable circumstance that Frodo made it to Orodruin and then only due to Gollum who, due to circumstance, had followed Frodo in order to get the ring, that it was destroyed.
There are plenty of other things that could be said to should have happend but the book wasn't written with those ideas in mind.