Yeahbut(c) there are variations:

caster,     |             |               | Lancaster
cester, | OE, W (<L) | Camp | Doncaster
chester, | | Fortification | Gloucester
caer | | | Caister
| Caerdydd,
| Carleon
-cester, -ster is a suffix
caer- is a prefix
The "-ster" sometimes simplified to "-ter", e.g. Exeter, Uttoxeter