N-Tier Architecture Is About Responsibility, Not Layer Count

N-tier architecture is a design approach that organizes software systems into layers, each with a distinct and isolated responsibility. The core principle is not the number of layers but rather separating concerns based on what changes, how often, and with what consequences. A typical system benefits from four layers: HTTP/API, application use cases, domain/business rules, and infrastructure. Each layer is aware only of the layers below it, ensuring that changes in one area — such as a database schema update — do not cascade unpredictably through the rest of the system. The goal is predictable change, clear boundaries, and a codebase that can grow without collapsing under its own complexity.