All Malory tells us of Abblasoure can be found in XIII, 14:
And at the last it happened [Galahad] to depart from a place or a castle the which was named Abblasoure; and he had heard no mass, the which he was wont ever to hear or ever he departed out of any castle or place, and kept that for a custom.
Could it be that Galahad heard no mass here because the inhabitants, if any, were Pagan? We are not told Galahad’s emotions during his visit. He seems to have gotten along peaceably; this may have been early ecumenism, or he may have found the place deserted, even ruinous. After leaving Abblasoure, he prayed at a desolate old chapel on a mountain, where a heavenly voice directed him to go to the Castle of Maidens and destroy the wicked customs there.
Rhayader Gwy, in Radnor, Wales, is handy both to mountains and to the Castle of Maidens if Llanidloes is accepted as the latter.
Source
Le Morte Darthur | Sir Thomas Malory, 1469-1470