Cloud computing has become a foundational technology in modern information systems by providing on-demand access to computing resources such as storage, processing power, and networking over the Internet. Organizations increasingly adopt cloud platforms to enhance scalability, flexibility, and cost efficiency. However, the migration of sensitive data and critical applications to remote infrastructures introduces significant security challenges. These challenges include data privacy risks, multi-tenancy vulnerabilities, regulatory compliance concerns, and lack of direct infrastructure control. This paper presents a comprehensive analysis of cloud security architecture, including service models, deployment models, architectural layers, and major security threats. It further discusses protection mechanisms such as encryption, identity and access management (IAM), accountability frameworks, secure key management, and compliance monitoring. The study aims to provide a structured understanding of designing secure cloud architectures while balancing performance and operational efficiency.