Don't forget the Sigil Towers. Although instead of keeping the two planes seperate, they link the planes together.
The term "Towers" has a more spesific meaning here then you seem to have thought, thoguh that parallel is interesting.
And of Nu-Hatta's list, only Orichalc is somewhere else, namely Yokuda.
Not any more. Now its gone (hence why the Redguards moved to Tamriel).
Usually when a tower is destroyed, bad stuff happens it its respective continent Sinking and freezing over being two prominent examples.
I guess that means Akavir's tower was destroyed before the first era...