Canterbury was walled by the Romans around 300 AD. This has been consistently the most important of the city's gates as it is the London Road entrance and the main entrance from most of Kent. The present towers are a medieval replacement of the Roman west gate, rebuilt around 1380.
There was a gate here at the time of the Norman conquest, which is thought to have been Roman. From late Anglo-Saxon times it had the Church of the Holy Cross on top, but both church and gate were dismantled in 1379, and the gate was rebuilt by Archbishop Simon Sudbury before he died in the Peasants' Revolt of 1381.
It has been suggested that it was built primarily as an entrance for pilgrims visiting the shrine of St Thomas Becket at the cathedral. However the rebuild as a defensive status symbol was paid for partly by Sudbury and partly by taxation for military protection against expected raids by the French.
It has been suggested that the spiral staircases were built to the disadvantage of defenders going downstairs who would have had to fight invaders climbing up to their right: difficult with a one-handed sword in the right hand.
Source & More Information: Wikipedia, Westgate, Canterbury, //en.wikipedia.org/wiki/Westgate,_Canterbury