http://www.bafra.org.uk/html_pages/articles_casters.html
From the text:
The origin of the castor goes back to certainly the early 16th century when, 'baby cages' and invalid chairs are known to have been equipped with wheels. They were used in England certainly towards the end of 17th Century and by 1690 there was an established castor-making trade in London. Castors, to begin with, were simple hardwood wheels mounted on horizontal axles but by 1700 vertical spindles mounting jaws in which horizontal axles and wheels were carried were in use. The wheels were of wood, probably boxwood which, by the 1730s, tended to be replaced by leather wheels and then brass wheels.
The origin of the castor goes back to certainly the early 16th century when, 'baby cages' and invalid chairs are known to have been equipped with wheels. They were used in England certainly towards the end of 17th Century and by 1690 there was an established castor-making trade in London. Castors, to begin with, were simple hardwood wheels mounted on horizontal axles but by 1700 vertical spindles mounting jaws in which horizontal axles and wheels were carried were in use. The wheels were of wood, probably boxwood which, by the 1730s, tended to be replaced by leather wheels and then brass wheels.