Information Systems Security Architecture (ISSA) has become a crucial branch and research focus of Information Security Science, and one of the key technologies in guiding the R&D of large-scale secure system (or security products) in the past ten years. The conceptual connotation of "Architecture" and the significance of Security Architecture have been proposed. The variety definition of ISSA is introduced, and its development trace is systematically overviewed. Based on the above, two different research methods and train of thoughts have been revealed. Then, the idea that "ISSA is the positive result of practicing systematic thinking in the Information Security field of investigation" has been clearly proposed. The disciplinary nature, the methodology of ISSA, and the challenges in the development of ISSA in theory, technology, education, practice and application are briefly discussed. Finally, the notion that Each Unification, of the Subjects like Information Security and Science of Systems, started from the basic science, should be the major trend and effective approach to radically solving the problems in Information Security is proposed.