Lancaster literally means ‘the Roman fort or town on the Lune’. The ‘caster’ comes from the Latin ‘castra’ for military camp. It is one of a small number of Latin words that were used by the Anglo-Saxons and for them meant that it was either a Roman site or an old fortification (most usually both).
It also became ‘chester’ or ‘cester’ depending on the local dialect, but around here the hard ‘k’ sound prevailed in the end. This may well be linked to Scandinavian influence. The Welsh equivalent is ‘caer’ as in Caernarvon and Caerdydd (Cardiff).
In the Domesday Book of 1086 Lancaster was referred to as ‘Loncastre’ just as the Lune Valley was Lonsdale.