I put this in for the sake of completeness and it it is worth seeing it if you go to see the other sights at Castlefield, as you certainly should. However I could not claim that it is one of the principal Roman remains in Britain.
The area is known to have been fortified early in the existence of Roman Britain by Agricola. This, however was a wooden building and the present building was probably the fouth fort to be built on the site, or rather a partial reconstuction from the ruins. Not only was it a good defensive site against tribal uprisings but it lay about half way between the important Roman towns at Chester and York.
If you disregard the fact that it is a reconstruction, the area as it appears now is quite educational, showing the foundations of some civil buildings as well as reconstructions of ahospital, soldiers quarters and the commander's house.