[ffcmike]

I'm somewhat surprised with your feeling about linearity, as when you factor in certain levels only being accessible once the water is lowered, it is straight forward from the start, with the exception of one right turn you can take if you kill all of the rats for the key where that door is. This level is then also not possible to progress through without the big key.
Once the water level is lowered it's then a matter of re-visiting the same levels you already passed through, with it then being possible to enter an entirely new set of lower levels at the same place you began.
It shouldn't really be a matter of figuring out what path to take, but how they combine to enable you to obtain the big key.

I'm wondering if you noticed the book in the level before the rat boss?
There will always be some trial and error involved in defeating the boss but I think this book should atleast in-still the idea that you need to tire it out.

Maybe I should have elaborated some more on save points, they can actually be set within any bed rather than being something you find within specific places.