More of a question than a theory, but bear with me...

At the end of the SI quest line you meet Dyus of Mytheria, the custodian of all of Jyggalag's knowledge. He says that in order to mantle the Daedric Prince of Madness you'll need the Staff of Sheogorath. If I understand correctly how Towers eork in Mundus, could the Staff of Sheogorath be considered a tower of Sheogorath's plane of oblivion?

Nahh. Not a tower. Maybe a stone. Seems like kind of a stretch though.

I'm pretty sure the Tree of Madness would be the tower equivelant.

Oblivion is littered with Towers, thus the insane amount of Sigil Stones.

Each pocket of a Daedric Prince's realm acts like Mundus to some extent but on a smaller scale.

I'd assume that Sheogorath is the tower of the Shivering Isles. Each realm is held up and has it's "rules" made by the ruling prince. I was still wondering about the Towers/Sigil Stones in Dagon's plane.