Indeed. This is why many castles have a fortified well with a locked cover on it in the basement store rooms. They make it hard to poison because it is an obvious weak point. If a besieged castle has no drinkable water it is game over.

Actually that would make for an excellent war time adventure for an adventuring group. Get into the besieged castle and poison the water supply. It would be a nice variant of the slip in and open the gates adventure theme and could be done by low level characters.