Where should you add layers in an application architecture?

In a typical layered architecture you separate concerns into distinct layers: presentation (UI), application/service, domain/business logic, data access, and infrastructure. When adding a new layer, you insert it between existing layers to isolate responsibilities and keep each layer independent. For example, if you need caching, place a caching layer between the service layer and the data access layer so that the service layer remains unaware of caching details. If you need an API gateway or authentication service, add it at the front of the presentation layer. The key is to expose only well‑defined interfaces to adjacent layers, maintain separation of concerns, and avoid tight coupling. This approach improves testability, maintainability, and scalability.