A Castle Windsor developer primarily works with the Castle Windsor, a .NET framework for building applications. They design, implement, and manage software applications using the Castle Windsor's Inversion of Control (IoC) and Dependency Injection (DI) principles. Their responsibilities include configuring the container, managing component lifestyle, understanding component registration, and integrating with different frameworks. They also troubleshoot and fix issues related to the Castle Windsor container. Their work contributes to the software's modularity, testability, and maintainability, as they ensure components and services are loosely coupled yet highly cohesive. They often collaborate with other developers and stakeholders in their work.