When Sir Ironside besieged Castle Dangerous, Lynette came to Arthur’s court at Caerleon to find a champion for her sister. Lyonors’ castle may perhaps be found by attempting to retrace Lynette’s return with Gareth Beaumains. After leaving Caerleon, Gareth fights and kills two wicked brothers at a “great river and but one passage”. The Severn makes a horseshoe curve at Newnham, below Gloucester, and continues to make little bends between Newnham and Gloucester. Malory’s “great river and but one passage” might be somewhere here.
Continuing on, Gareth fights and kills the Black Knight of the Black Laund (Percard), that same day, in a valley. Still, apparently, the same day, he defeats the Green Knight (Pertolepe) (not to be confused with the title character of ‘Sir Gawaine and the Green Knight’), who lodges him and Lynette for a night and next morning escorts them partway through a forest. This day Gareth defeats the Red Knight (Perimondes) by a white tower at the Pass Perilous. I would guess this to be a mountain rather than a river pass, and put it in the Cotswolds, perhaps at Stroud. The Red Knight also lodges them overnight.
The following day Gareth defeats Sir Persant of Inde near a “city rich and fair”, which is “seven mile” from Castle Dangerous. Castle Dangerous is later described, in a cry made throughout Britain, Ireland, and Brittany, as “beside the Isle of Avilion.” Sir Persant’s city might be Tetbury or Chippenham, and Castle Dangerous seven miles beyond that, toward Avilion.