moved Anonymous comments here
regarding the following page content:
Dull Rat's Ring - when obtained from Lord Rydell in 3-1- unlocks the door to the second floor of the Tower of Latria (This is not 100% true. If you release Rydell and then kill him before you unlock the door it will remain locked.)
I have verified this 100% now. If you go back after unlocking the door Lord Rydell will not be there anymore so it seems that it is he that unlocks the door and not possession of the ring itself.