The Magic of the Elder Folk by Geoffrey Monck is a tome which contains information on, among other things, portals. It is cited in the Ars Magica.
And Geoffrey Monck, probably the greatest authority in the field of teleportation, the author of the work entitled The Magic of the Elder Folk, which is the opus magnum of knowledge about elven teleporters, writes that the Tor Lara portal leads to the Tower of the Swallow, Tor Ziraeal—
pg. 330 - 331, The Tower of the Swallow (UK edition)