This article reframes security as an architectural property of regulated financial services data platforms, not a bolt-on set of controls. It argues that true security lies in preserving temporal truth, enforcing authority over data, and enabling defensible reconstruction of decisions under scrutiny. By grounding security in threat models, data semantics, SCD2 foundations, and regulator-facing narratives, the article shows how platforms can prevent silent history rewriting, govern AI safely, and treat auditability as a first-class security requirement.