There are no ‘essential subjects’ required for you to study Architecture. However most universities insist that you have a pass in Mathematics. Art can be an advantage, as can Engineering, Graphics and Design (EGD). Physics can also be informative.
To use the architectural titles and provide architectural services in South Africa, it is a legal requirement under the Architectural Profession Act 44 of 2000 to be registered with the SACAP. If you are not registered you cannot represent that you are an architectural professional, which includes stating or implying that you are an architectural professional, practicing as an architectural professional or undertaking or are willing to undertake work as an architectural professional.