Maserfelth was where Penda of Mercia in 642 defeated Oswald of Northumbria, who was killed in the battle. It was a dramatic event, emphasized by Bede, who describes miracles later occurring at the spot, which he took as that of a martyrdom. Despite these clues, its location has been obscure. Yet the traditional identification with Oswestry (National Grid Reference (NGR) SJ 2929) in Shropshire was rightly rejected by the place- name scholar Margaret Gelling. A better answer can now be given on the basis of a communication to Ferdinand Holthausen by Max Förster (1869– 1954) of Munich, who derived Maser- from Welsh. Maser- , relating to place names near Welshpool, Powys, thus apparently denotes the - felth or plain of the Severn close to Forden, where a Roman road enters Wales. As for the battle's Old Welsh name Cocboy (where coc- perhaps denotes a hillock), this seems to be Castle Mound (NGR SJ 2301), a conspicuous outcrop in Forden parish.

Now for the details. The Battle of Maserfelth, where the pagan king Penda killed Oswald of Northumbria, has been at once famous and obscure. Thanks to Bede, it has never been forgotten; but why it was fought and what happened on the field of conflict remain obscure. Its importance was still recognized by Britons and Saxons alike. Even though nothing shows that the Welsh took part in the engagement (a struggle for mastery among the English), Annales Cambriae has an entry for 644, ‘Battle of Cocboy, in which fell Oswald king of the Northumbrians and Eoba king of the Mercians’. Difficulties begin when we try to locate it. Plummer had notes on Bede's account of the battle, and referred to late traditions of it at Oswestry, near Offa's Dyke. At the same time Maserfelth was explained tentatively as ‘open space of maple trees’, despite phonological difficulties for maser (which, if signifying ‘maple’, should not have a as first vowel) and absence of the element from other English toponyms, where the word for this tree is mapuldor.

On the location at Oswestry, Sir John Lloyd regarded Old Welsh Cocboy as counting neither for nor against it. But he saw ‘much in favour’ of accepting Maserfelth as Oswestry, because ‘Bede's account of the miracles which signalized the spot’ implies somewhere ‘in a wild region sometimes visited by British wayfarers’.