Once Rome built a fortress in the west of Britain. It was called Deva Victrix, goddess of victory, and indeed there remains today a shrine to Minerva there. It was the sometime home of Legio XX, and is located in what is modern day Chester, England.
Some believe that this was the "City of Legions" where Arthur fought his ninth battle, although Caerleon in Wales is a competitor for that honor. Caerleon also has a significant Roman fortress. The University of Wales at Newport has built a working 3-D model of the Roman works, which you can explore to get an idea of the scale of the fortifications.
The map is pretty neat, even if one can easily think of significant improvements that could be made -- it would be nice to have some sort of hypertext tagged to the building objects, for example, that would lead to explanations of just what they were and what historical or archaeological sources are at work in our understanding. A bibliography or a list of recent research into the works would be welcome as well.
Still, even at this early stage, it's pretty nifty stuff. They're apparently putting one together for the Newport Ship, as well. That one -- a 3-D model of a fifteenth-century sailing vessel -- should be fun to play with.