What exactly are the Elemental Watchtowers in the Enochian paradigm?

Are they considered to be the entirety of the Elemental Planes?

Are they specific "outposts" in the respective Planes? e.g. Is the Watchtower of Earth an outpost somewhere in the hinterland of the Elemental Plane of Earth?

Are they entirely something else - and, if so, what?

The watchtowers function as both conduits, waystations and controls for the flow of elemental energies and beings into and out of the material world; especially in relation to the Lunar sphere, which has a special relavance in Enochiana. This is Donald Tyson's view anyway, and thusfar in my experience, it has held true.

I performed the Golden Dawn Watchtower Ritual with a friend who is psychic functioning as my seer and we had a beautiful experience. The watchtowers are both energies and living beings, made up of the component entities from the banners/tables, and yet more than the sum of their parts (as with all group dynamics/egregores). As we invoked their power, my seer saw the three banner angels on each of their ramparts raise their wings, as well as the entire tower shifting and sending out what he described as 'scarves' of energy. When I completed with the metainvocation of ELXRP COMANN TABITOM, the scarves combined in a Captain Planet fashion, flew upwards into the sky (turning it a bright color) and invoking the presence of Deity. With God then presiding and in the presence of the angels, we were then able to communicate and make our request.

We found the watchtower angels to be very friendly and encouraging. Their language is energetic - a song far beyond the comprehension and limitations of our language, yet able to be scaled down for a human interpretation. They wished us peace, and even called me a little brother. I'm not quite sure what they meant, but it was very reassuring.

From my experience I would have to agree with you here about the spirits of the watchtowers. They are very regal indeed, though they are above the king.

As others commented in this post, the Watchtowers are both places and entities, much like many things in magick (the same goes for the Governors). They permeate the entire physical world, and yet are above it, and it could also be argued that they are also part of ourselves.

LLLSHJ,
Yechidah.