Category A, B. C and D refers to the regime by which prisoners are managed, not the prison. All prisons, apart from open (Category D) prisons, are built to common standard, with certain modifications where additional security requirements are required and appropriate to the category of prisoner housed there.
Category C are prisoners who cannot be trusted in open conditions but who do not have the resources and will to make a determined escape attempt. The security in prisons holding these prisoners will include a secure fence/wall in accommodation blocks.
Category D are prisoners who present a low risk, can reasonably be trusted in open conditions and for whom open conditions are appropriate. There are currently no direct build standards for Category D conditions.