I am not sure if I am now wording it correctly, but I feel like part of the feel of both Emelental and Lord of the Rings is the mystique. Orthanc is amazing because no builders who still live could construct a tower like that again. If you however can build this tower yourself then the mystique is gone even though he functionality would still be the same. In the end if you care little about the mythical feel then being able to build these fortresses would be perfectly fine. Limiting one per empire or making them very expensive would be fine.
I agree that being able to just build one whenever doesn't seem quite right, but I do want to point out that Barad-dur (a much cooler tower, IMO) wasn't built by some ancient, long lost builders.
This brings me to a question I have. How much can we "parallel" Lord of the Rings? Will we be able to infuse essence into our keep/tower/fortress/etc to make it nearly indestrucable? Can we infuse most/all of our channeler's essence into an object (ring, armor, magical boot
etc) and use it to increase their power, but at the cost of if it is destroyed, we are destroyed, or another channeller can capture it and use it against us? Will we be able to reinact the entirety of LotR from the first war of the ring/last alliance all the way to the final defeat of Sauron? Can you tell I'm a huge LotR fan?