Chester (placename element)

The English place-name Chester, and the suffixes -chester, -caster and -cester, are commonly indications that the place is the site of a Roman castrum, meaning a military camp or fort, but it can also apply to the site of a pre-historic fort. Names ending in "-cester" are nearly always reduced to "-ster" when spoken, the exception being "Cirencester", which is pronounced in full. The pronunciation of names ending in -chester or -caster is regular.

...
Wikipedia