what does the "ster" ending in dorchester, leicester and lancaster mean?

Question

why do so many english town names end like this?

Answer 1

Jennerfer... is right on the money. And Latin castrum led to Norman French and then English "castle" and Old French and then English "chateau".

Answer 2

The place-name suffixes chester, cester, and caster derive from the Latin word castrum (fortified place).

DORCHESTER:
Two miles southwest of the town centre, sat on a steep chalk hill, are the remains of a large Iron Age hill fort Maiden Castle, which was one of the most powerful settlements in pre-Roman Britain. The fort was important to the Romans and the Saxons, whose invasions of Dorset weren't seen as complete until they had captured the hill. No actual fort exists at the site now but the impressive ramparts surrounding the hill still exist.

Likely conclusion: Dorset + chester = fortified hill at Dorset

LEICESTER:
According to Geoffrey of Monmouth, the mythical king of the Britons King Leir founded the city of Kaerleir (Leicester). He was supposedly buried by Queen Cordelia in a chamber beneath the River Soar near the city dedicated to the Roman god Janus, and every year people celebrated his feast-day near Leir's tomb. William Shakespeare's King Lear is loosely based on this story.

Leicester is one of the oldest cities in England, with a history going back nearly 2000 years. The Roman city of Ratae Coritanorum was founded in AD 50 as a military settlement upon the Fosse Way Roman road. The city was named after the Corieltauvi, the Celtic tribe that dwelt in the area before the Romans arrived. Ratae Coritanorum grew into an important trading and military centre and one of the largest towns in Roman Britain.

LANCASTER:
The etymology of Lancaster is derived from its large and imposing castle (the "caster" part), and the river that runs through it, the Lune (a name deriving from old Celtic sources). Language evolution from the old name of Loncastra, the castle-town on the Lune, gives the modern name of Lancaster.

Answer 3

As the first answer points out, they are all former Roman fortifications, and you get similar place names all over Europe, "Castello...." in Italy, "....kastel" in Germany and Austria, "Chateau de ..." in France, where the meaning changed and was used for (new) settlements around a country manor well into the 16.th century.

Answer 4

Same as the STER starting with DUMP.