ok i'm going from memory here, bear with me.
1st you need to find the arch way...
if you start in the room w/ the five doors that you can't open (and they don't ever open imo)
and you get to the timer blockade in the west - you need to get past that, try resting for a day...
the archway room should be past that blockade and the passage will *ahem* snake to the north.
then you can drink the potion, flip the switch - this lowers the drawbridge.
over the drawbridge after more running around eventually you will find the fourth pool of fresh water, once all four have been poisioned the hydraulic door will open.
then things will get even more frustrating.
