Doncaster is one of the more minor cities of the North of England, known by most as just one of the stops down to London on the GNER line. It has reknown as a centre for horse racing and not much else, but has a very ancient and venerable history stretching back to Roman times.
Romans
One of the oldest cities in Britain, founded by the Romans in AD71. (Around the same time as York, which is about 25 minutes North by train.) On the banks of the river Don they built Fort Danum, a name which has been bastardised over the years to become Doncaster - the fort (-caster) by the river Don (Danum). Fort Danum lay on one of the important Northern roman roads, Irmine Street, which continued north to York and on into the moors.
Saxons
During the reign of the Saxon kings, Doncaster (along with York) became one of the first strongholds of christianity in the North of England. Both cities were ruled by King Edwin, monarch of Northumbria, whose stirling work to bring everyone to salvation was completely undone after his death in AD633 when Northumbria reverted back to paganism.
Norman
Doncaster is mentioned in the Domesday book (written after the Normans' arrival in England) as being a soke of Hexthorpe, which is now a suburb of Doncaster. The Norman army built a very fine castle at Conisburgh, another of Doncaster's suburbs. This castle is one of the best examples of a circular keep in Europe and was used by Sir Walter Scott as one of the settings for Ivanhoe.
From here, Doncaster's history becomes a little thinner on the ground, and nothing noteable really happened until the late Middle Ages. The city was a thriving market town throughout the Medieval period, but many of the estates and town houses representing this boom time have been swept away by modern development in the 1960's and 70's. Horse racing became Doncaster's main attraction from as early as 1595 and continues to be so today. Doncaster was also noted as having one of the richest coal seams in Britain, before the mines were closed down in the latter half of the twentieth century, decimating local jobs.
Today Doncaster offers good shopping facilities, infamous racing meets and several notable buildings which are worth a look, but the destruction of so much of its structural history in the last century means that to most, it's just one of the stops on the GNER line on the way down to London. Sad really.
See:
http://www.doncasteronline.com/donc_online/about_donc/index.html
http://www.doncasternet.com/History.htm
for more information.