Mór-fæsten

According to the Old English Dictionary:

es;

mór-fæsten

n. A place secure from attack from the swampy character of the country :-- Hé (Alfred) lytle werede uniéþelíce æfter wudum fór, and on mórfæstenum. Chr. 878; Erl. 78, 34. mor-fæsten