The Towers are definitely real, and their presence or absence makes a huge difference in the reality of the world. If more than two, certainly no more than three, were to fall, the barriers between Mundus and Oblivion would become extremely weak. If four or five fell, things would completely fall apart and there would be no barrier at all. They're not just concepts or titles. They have real power.
Ah, this is a subtle point that I have only recently begun to understand, but I will try to submit what I have heard. My understanding is that a new Tower can be made (that's how they were made in the first place), but it is not an easy task. They must be physically and magically copies of the first Tower (not cookie cutters, but they must be in Tower shape), they must have a Stone, and I'm fairly certain they must be recognized by the divines. This is why I would think Martin could make a new Tower, and I'm sure a similar process would need be undertaken to make the other seven. I think the most recent was Numidium, and I think Anumidium replaced it since it was pretty much the same thing. Only difference was the Stone (can one Stone serve two Towers? If Numidium used the Heart? I suppose that's why Anumidium used the Mantella. Dwemer disappearance theory perhaps?)
Their power comes from their Stones and their shape, I believe. The Stones might be powered by the shape, but I'm a little fuzzy on that. The Stones draw creatia from Aethirius, and along with keeping the laws of nature (earthbones) intact and Mundus together, this creates the Liminal barrier. But I believe the power of the Stones (just theorizing here) comes from their circle shape, which is why White Gold is so important at the center. As you know they form a wheel, the eight on the outsides representing different Aedra (Adamantia is Akatosh, Hrothgar would be Kyraneth, not sure if Red Mountain counts as Lorkhan, and I don't know the others), and Gold is representing Mundus (Amulet of Kings represents Akatosh and Lorkhan :shrug:) It's all in the intercept (shouldn't have to link anymore)
Indeed, and most were at half or no power during the Oblivion crisis, though White Gold was most important. Numidium was gone from Daggerfall, Red Mountain from Morrowind, Orichalc is gone, Hrothgar is missing half of it, not sure about Falinesti or Crystal, the Khajiit's is too obscure to say (though the Mane's death was important), etc.
And of course the reason the shape is important is because their wheel shape symbolizes the Wheel, in essence the echoes of the zero tower make an echo of the Tower (I). Everything fun is always connected in Tes
EDIT: Indeed, I was simply concerned because that leaves several Aedra out. I can't imagine what Mara's, Dibella's, Julianos', etc. represent. Heck, the Khajiit seem more Sheogorath or Lorkhan inclined than anything (Lorkhan needs to stick to Mundus where he belongs
)