Mythen Keep (Ars Magica 5)
A Hermes Portal is a very powerful ritual that forges a gateway between to physical locations. It is a spell of 15th magnitude (ReTe 75; Mercurian Ritual) found in ArM5, page 156.
With all of that being said, below is a listing of known Hermes Portal locations.
Famous (Imfamous) sites:
Harco / Durenmar
Harco / Blackthorn
Durenmar / Diedne (reported lost during the Schism War)