Muntgumeri, Montgomery 1086, Monte Gomeri 1166, Monte Gomerici c.1190, Muntgumri 1094, (12g.), Mongomery 1211, 1268, Mungum(er)y 1215, Mungumberi 1227, Muntgumeri 1226-8, Monghomeri c.1283, Moungomery 1291, Monte Gom’y c,1291, Mungumbria 1214, (13g. Ddiw.), Mont Gomery 1312, Mount Gomeri 1313, Mountgomery 1313-18, c.1679, Moungombry 1393, Mountegomery 1493.
(o) gastell baldwin c.1170 (13g.), castel Baldewin 1257 (c.1286), Castell Baldwin 1231 (a) Kastell Baldwyn 1267 (c.1400), Castell Baltwin 1115 (c.1400), castell baldwin cyn 1220 (15g,). Trefaldwyn1440, c.1570, yn h6r bald6yn, Tref uald6yn 1447-89, Montgomerike, in Walche Treualdwine 1536-9, Trefaldwin c.1566, Treualdwyb 1584, Tre valdwyn c. 1560-90.

The original location for Montgomery was the ‘Hen Domen’ (‘Old Mount’), a Motte-and-bailey castle next to the river Severn, which was named after Sainte-Foy-de-Montgommery or Saine-Germaine-de-Montgommery (Calvados) in Normandy. The name was given to the new castle, “New Montgommery”, which was built in 1223-4.
In Welsh it seems that ‘Castell Baldwyn’ (‘Baldwyn’s castle’) was the original name for the place, hence the name of town being ‘Trefaldwyn’ (‘Baldwyn’s Town’). The name apparently refers to either Baldwin de Bollers (13Cent.) or, more likely, to Baldwin the son of William of Hodnet who we find first in 1223 “in the service of the Lord King at the new castle … outside Mungumeri”.
This Baldwin held land at Chelmick and Hope Bowlder in Shropshire, and mills on the Severn. It is possible that he was related to the first Baldwin and it is likely that he was the Baldwin who was killed in 1257 by the supporters of Llywelyn Fawr (Llywelyn the Great).