but others are set within the first and even into the second Empire.
A lot of them are more recent than that.
There seems to be an architectural distinction between the really ancient ones (i.e. Yngol's Barrow) and more recent ones, such as Hillgrund's. The older ones seem to be built mostly into the rock, with walls acting as support, whereas newer ones have antechambers and are mostly man-made.#
They're becoming more similar to the Halls we see in cities.