lretone 1086, T(re)fwern 1234, Alreton 1245, Orleton 1255, 1281, Allereton 1263, Olorton’, Oleretun 1271, Trewern’ 1287, Trewern 1310, 1344, 1568, Trefwern 1354, 1360, Treffwern 1421-2, Trewern alias Olarton c. 1558-70

Trewern – 'Settlement at the alder-tree marsh'. Many early forms have Old English alra, genitive plural of alor, and tün, matched by Welsh tref and gwern.
Alretone 1086 was long misidentified with Cause, Shropshire, the site of a large castle and Lordship which reached into Montgomeryshire as far as the river Severn.