It is not always necessary to have this key as some of doors in the towers are already open. There are two Sigil Keepers in Oblivion.
If the Hero somehow manages to get the Sigil Keeper to speak with them he will only say, "Be quick about it, mortal." This can be done with very high Personality, notable for preventing the Daedra from attacking. Alternatively one can use an active chameleon effect of 100%, and using charm spells while he can't detect the Hero.