Maisbeli
A British field in which Hengist the Saxon sought to set an ambush for Ambrosius Aurelius.
Ambrosius heard of the ambush beforehand, and arrived prepared. Ambrosius won the subsequent battle, and pushed Hengist on to Conisbrough.
Notes
Maes, an open field, is a contradistinction to cae, which is an enclosed field. It sometimes signifies a battle-field. Cad ar faes is a pitched battle, and colli y maes is to lose the battle.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138