yea, i read a book recently, it included how some enemies would poison the water supply by tossing infected things in it, dung, etc. And also, as you said, it is easier to poison a well, as a river would just take the poison to its outlet, killing people/fish/animals somewhere else.
that would be coolYou can add fell-beasts or other monsters that inhabit the mountain, environmental conditions, or just the logistics of dragging an invasion force over it. If whatever passes are there, are easily defended (a la the movie 300) that would discourage a large invasion as well.
bring de mayhem!!!