Design an API architecture for a financial services platform that must support: 1) Different access levels for various user types (retail, business, admin), 2) Rate limiting and throttling, 3) Strong authentication and authorization, 4) Full audit logging of all transactions, 5) Versioning strategy for backward compatibility, and 6) Integration with third-party services. Include specifications for API documentation, testing approach, and monitoring strategy to ensure reliability and security compliance with financial regulations.