Sǽ-fæsten

According to the Old English Dictionary:

sǽ-fæsten

the fastness or stronghold which the sea constitutes:--Óþ ðæt sǽfæsten landes æt ende leódmægne forstód the sea was a stronghold which blocked the further passage of the Israelites, Cd. Th. 185, 24; Exod. 127. sæ-fæsten